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Thermoregulation in four species of nesting herons

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Thermoregulation in four species of nesting herons
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Ellis, Hugh Irl, 1944-
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English
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vi, 101 leaves : ill. ; 28 cm.

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Bird nesting ( jstor )
Birds ( jstor )
Body temperature ( jstor )
Cattle ( jstor )
Cooling ( jstor )
Eggs ( jstor )
Metabolism ( jstor )
Plumage ( jstor )
Solar radiation ( jstor )
Sun ( jstor )
Animal heat ( lcsh )
Body temperature -- Regulation ( lcsh )
Dissertations, Academic -- Zoology -- UF
Herons ( lcsh )
Zoology thesis Ph. D
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bibliography ( marcgt )
non-fiction ( marcgt )

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Thesis:
Thesis--University of Florida.
Bibliography:
Bibliography: leaves 94-100.
General Note:
Typescript.
General Note:
Vita.
Statement of Responsibility:
by Hugh Irl Ellis.

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THERMOREGULATION IN FOUR SPECIES OF NESTING HERONS



















By

HUGH IRL ELLIS












A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF
THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY








UNIVERSITY OF FLORIDA 1976


















ACKNOWLEDGMENTS


I would like to thank my committee for all their help over the

years: John Kaufmann who helped me get started on this project and who supported me when I needed it; Brian McNab who opened up the world of physiological ecology for me and whose constant stimulation helped make me a different kind of scientist tha'n I would have been otherwise; David Johnston whose wealth of ornithological information was indispensable and was given freely; and James Lloyd with whom Ispent many hours in my early years here refining my ideas about evolution and adaptatiTon.

This dissertation would never have been possible without the

help of Donna Gillis who typed for me, befriended' me, and led me through the maze of red tape that theses are heir to.

Herbert Kale provided me with many of my experimental animals and helped me gain access to Riomar Island.. James and Eliz abeth Wing provided easy access to Biven's Arm and comfort from fatigue and the elements. Jon Bartholic, in the Fruit Crops Department, helped me to procure equipment which was essential for measuring solar radiation at my nest sites. To all these people I am greatly indebted.

Thomas Emmel,,Chairman of Zoology, has recently been miy greatest source of encouragement. He has strengt-hened my resolve at critical times, in the preparation of this dissertation.

Too many people helped me collect data in the field for me to enumerate here. But I am especially grateful to Joyce Newman, ii









Cristina Palacio, Thomas Rudegeair, Laura Solomon, Ginny McCormack, and Mary Paulic. One person, in particular, must be mentioned. J. Perran Ross, my friend and colleague, spent months in the field and in the laboratory with me. His hard work, stimulation, and criticism have been indispensable.

Esta Belcher provided all of the illustrations in this paper. Any errors in these figures are my own. Frank Nordlie kindly criticized and read this paper.

The research for this work was supported in part by a grant from the Frank M. Chapman Memorial Fund administered by the American Museum of Natural History.




































iii














TABLE OF CONTENTS


ACKNOWLEDGMENTS . . . . . . . . . . . . ... .

ABSTRACT . . . . . . . . . . . . . v

CHAPTER

1 INTRODUCTION . . . . . . . . . . . .

2 METHODS AND MATERIALS . . . . . ... 4

Study Areasr .eas.............. 4
Field Techniques . . . . . . . . . . . 7
L..ratory Techniques ........ ...................... 9
Statistical Methods . . . . . . .. 11

3 NEST SITES AND SOLAR RADIATION ..... ............... ... 12

4 ADULT METABOLISM ........................ 20

Metabol I s 21
Conductance .......... .........................
Body Temperature ................................ WE
Energetic Implications . ..... .... .. ......

5 THERMOREGULATI-ON AT THE NEST ...... ................. 50

Incubat ion . .*....................... ,
Behavioral Temperature Regulation ... .............. 5
Development of Homeothermy ....... .................. 66
Color in Nest ing Herons .......................... 8

6 THE SIGNIFiCACE OF COLOR IN HERONS ... ................ 88

7 CONCLUSVONS ........ .......................... 92

REFERENCES . . .. 94

B OGRAPH CAL SKETCH .......... .......................... ...101











Abstract of Dissertation~ Presented to the Graduate Council
of the University of Florida in Partial Fulfillment of the Requirements
for the Degree of Doctor of Ph'*iosophy


THERMOREGULAThON IN FOUR SPECtES OF NESTING HERONS

By

Hugh Irl Ellis

June, IY76


Chairman: John H. Kau'rmann
Co-Chairman; Brian K. Meab
Major Department: Zoology

Temperature regulation in Louisiana Herons, Little Blue Herons,

Cattle Egrets and Snowy Egrets is compared to investigate any different thermal requirements of these birds during nesti ng. Louisiana Herons antd adult Little Blue Herons have dark plIumage; Snowy Egrets a~nd Cattle Egrets are white or nearly white. Louisiana horons nest consistently in the shade, The other three species nest without regard to solar input, hence may nest in the sun.

Metabolic rates of adults are compared. Louisiana Herons have a high M the two white herons have a somewhat lower Tthan expected from the ir weight, and Little Blue Herons have aM, only 2/3 that 00


to nestin n the snwith dark plumage in a hot env ironment I t is shown that low ~is character~is tic of dark birds that have been studied in such environmentsTempe ratu re regulations~ at the nesc is investigated with regard to inclubat ing. adulIts anid nestli ngs. A stable egg temperature of A' apperars to be independent of pa ren tal gotten Lveness at normal T


V









Little Blue Herons spend much less time incubating when exposed to the sun than do white Cattle Egrets. This behavioral response is thought to be a con.,ssuence. of their dark plumage. Nestling Tb is nearly 39'C for afl species regardles of age, brood size, or T as a result of 1) both parental and nestling behavior, which is described, and 2) the ability of nestlinys to dissipate heat: by evaporative means. Cooling curves are used to determine the development of homeothermy which in these herons is a function of age, not weight. Short-term homeothermy is achieved by 13 days, although there is some indication that Louisiana Herons achieve hmoohermy iater than the other three species. Growth rates are compared and all are tentatively considered to fit the logistic equation, althou h the Cattle Egret nearly conforms to the Gompertz equation. Possible t:hermal reasons are considered for the dark skin color in all four species and the white plumage of young Little Blue Herons.























vi














CHAPTER I

INTRODUCTION


Herons (Ardeiformes, Ardeidae) are nonpasserine birds with a wide range of size and distribution. Many of them breed in Florida; among these are four smallspecies all with relatively similar nesting habits. Three of these species are probably congeneric (contra AOU Checklist, 1957); the fourth is a recent Old World arrival.

The Louisiana Heron (Egretta [=Hydranassa] tricolor) is probably slightly larger than the other species. It has a bicolored pattern, being a dark gray dorsally and white ventrally. The Little Blue Heron (Egretta [= Florida] caeruZea) is the darkest bird, having a slate blue body plumage and rust-colored head and neck as an ndult. The Little Blue Heron is unique among these four herons in having a marked difference between adult and juvenal plumage. immatures have a white plumage which toward the end of their first year gradually molts to the adult plumage. Snowy Erat.. (g~ ectta thuia thUZa) have a completely white plumage. The Old World Cattle Egret (BubuZcus ibis) probably arrived from Africa orn tie northeast coast of Sath Amrica in the nineteeth century, was first seen in Florida ir 1941, and only began breeding in Florida in 1953 (Palmer, 1962). The Cattle Fgret is basically white with patches of buff cilor on the hood, back, and breast during the breeding season.

The inclusion uf Littlic Vue. H rcns d Louisiana Herons with Snowy Egrets in e ger us E.,tta h: as only recentiv been proposed










(Dickerman and Parkes, 1968, Parkes, pers, comm.). Dickeran and Parkes noted hybridization of Louisiana Herons with Little Blue Heron:s and with Snowy Egrets, They als o cited a note by Sprunt (1954) of a hybrid from a Little Blue Heron and a Snowy Egret cross. Curry-LindahI (1971) suggested a reclassification of the Ardeidae on the basis of behavior and ecology. He too suggested that Little Blue Herons, Louisiana Herons and Snowy Egrets all belonged in Egretta,

All four species of herons have mainly tropical to subtropical breeding distributions, although a larger subspecies of the Snowy Egret (Egretta thula brewste;i) nests in several of the western states of the United States (Bent, i926; Paimer, 1962). The Cattle Egret has recently invaded temperate areas in North America (Palmer, 1962; Weber, 1975), Australia (Palmer, 1962), and southern Africa (Siegfried, 1966a). The Cattle Egret invasion of temperate zones has mostly occurred since 1920 and is apparently a response to increased pasture land (Siegfried, 1966a). The tropical/subtropical distributions alone of these intermediate-sized nonpasserines makes them an attractive subject for study: most nornpasserines studied are temperate species.

All four of these heronrs nest together in mixed species heronries

throughout Florida arcithe rest of the Gulf coast states. In Florida they nest in Spring when solar insolation is most intense. Their different colors under strong solar radiation must affect: their nesting patterns in some way, yet no one nas looked at nesting in he.rons from a thermal viewpoint. In fact, relatively little is known about their thermoregulation at all. Benedict and Fox (1927) reported rates of metabolism for the :ean. ue evn (Ard,a he:w&c. and th:a A.er an Bi ttern (Bota"urus









ZentigLrtoas); Siegfried (1969) measured the existence metabolism for Cattle Egrets at one temperature which was below their thermoneutral zone: Bartholomew and Dawson (1954) did some preliminary work on thermoregulation in nestling Great Blue Herons; and Hudson et al, (1974) described the development of endothermy in nestling Cattle Egrets. No comprehensive study has been done integrating the thermal requirements of adult and nestling herons with their actual conditions in the field. This study attempts to explain the thermoregulation of herons in the context of nesting in a hot environment.













CHAPTER 2

METHODS AND MATERIALS

Study Areas


Little Blue Herons, Louisiana Herons, Snowy Egrets, d Cattle Egrets were studied from spring 1971 through 1975. ;Most of the field work was done on liven 's Arm, a lake in Ga inesville, Alachua County, Florida. Some solar radiation measurements were made oan Riomar Island in the Indian River, at Vero Beach, Indian River County, Florida.

Siven's Arm has been described by Carr (1942), Rudegeair (1975), and Nrdlie (176). It has about 65 hectares of open water (Nardle, 1976) and is surrounded by pasture and swarpy woodland. The current herorry has existed on the northeast side since the early 1960's and probably represents the same heronry described by Jenni (196) at Lake Alice 1.5 miles northwest from i958 to 1960 (Nordil ., pers. comm.). Janni speculated that the Lake Alice heronry had originally come from Biven's Arm in 1948. In 1972 some of the narons from the northeast side of alven's Arm built nests on a series of five small islands on the southwest side of the lake. The islands were numbered sequentially 1 through 5 starting at the north. by 1573 as many as 40 percent of the herons on Biven's Arm nested on the southwest side of the lake. The southwest islands were arcnored, and only on their outskirts was the vegetation partiv filoes.ng or else slightly s .ergcd. Some, if not all, of the islands ware at one tirre f o. ting free in Biven's Arm, presumably havi ng been torn by wies fVon another part of the srel in. Aeria


'4









maps from 1967 do not show them on the southwest shore of the lake. The northeast heronry, on the other hand, is not clearly separated from the shoreline. A swamp of partly submerged vegetation, narrow waterways, and floating islands spans the few hundred meters between pasture and open water. The heronry occupies that part of the swamp closest to the open water, rarely penetrating more than 75 meters from it.

The four species of herons studied on Biven's Arm nested primarily

ir, red maple (Acer rubrim) and elder (Sambucus simpsonii) which they shared with Anhingas (Anhinga anhinga) and, in most years, White Ibis (Eudocimuans aZbus), Some buttonbush (Cephalanthus occidentalia) was intermixed with the red maple and elder in 1971, but virtually all of it died that year, and it only began to reappear in 1974. Floating

cut-grass (Leersia hexandra) was abundant at the margins of the heronries arnd also scattered throughout most of the northeast heronry along or in the waterways. Pennywort (Hydrocotyle spp.) existed in spots at the water's edge at the heronry sies. By 1973 and especially 1974, very large growths of water hyacinth (Eichho:nia crassivs) covered the water surface adjacent to the heronries.

Several other avian species, nested within the heronries:

Green Herons (BLutorides virescens), Common Gal1 lnules (GAZPiul horopus):; Purple Gallinuls (Porph; .ula ,:tn,infca); and Least Bi tterns (.Tb ~,,c hs ezilis.: whose numbers increased dramat i cally in 1975 perhaps due to the expansion of water hyacinths. A very few American Coots (Fuica americana) and Black-crowned Night Herons (Nyctico;rc ngeicorax) were occasional ly seen in the heronry, bt without evidence of breeding, at least during the spring and early





6



summer. In 1974 and 1975, Double-crested Cormorants (Phalacrocorax auritus) and a very few Great Egrets (Casmeroidus albus) roosted at the southwest heronry at night. Boat-tailed Grackles (Cassidix major) and Red-winged Blackbirds (Agetaius phoeniceus) nested on the fringes of the heronry and less often within the heronry.

The Biven's Arm heronries were somewhat insulated from mammalian predation by water. However, a raccoon (Procyon lotor) was sighted in a tree in the northeast heronry on April 15, 1972, emptying two Little Blue Heron nests. Another raccoon was seen on the southwest shore next to island 2 on May 9, 1973. Crows (Corvus spp.) were often seen at the height of the breeding season. Owls were apparently active predators in the heronry: owl pellets were found with piles of white feathers on the southwest shore. Alligators (AZZigator mississippiensis) presumably ate young herons that fell out of their nests and perhaps adults drinking water below their nests. No snakes were ever seen in the heronries, Possibly Black-crowned Night Herons are occasional predators (Rudegeair,



Riomer island is a spoil island with a large heronry described by Maxwell and Kale (1977). It has large Australian Pine trees ue, a.", equisetifolia) but the rookery occupies only mangrove t-e-s, pr i mar i y b ack mang rove (Avicnnia nitida) and wh ite mangc.ve (Lauouteria racemosa). Most of the birds nesting with the four speci es studied here are. also ardeIds, the major exception being Brown Pe icars (Pelicanus occi denl;is) wh ich maIke up 11 percent of the bre-din b d rds (IMaxwell and Kela, 1974) Predation on Riomer i vsind was by Fish Crows CCorvus occi.gs) and potentially rats

,.us. rm>a .. ..ccor.ina to Maxwell cand Kale (1977);





7



Field Techn iaes

Observations at Biven's Arm were made from a rowboat, a tower, or occasionally from shore (only for the southwest islands). A 15 foot high tower with a 4-foot square observation platform 12 feet high was attached to a raft 10 feet by 15 feet in size. The raft could be moved by poling with a 16-foot pole which was equipped with a "duck bill' at one end to allow bracing against the soft mud bottom of Biven's Arm. By shrouding the top of the tower with burlap a movable blind-on-stilts was effected. Observations were made using 7 x 50 binoculars.

Nests on Biven's Arm were approached in a rowboat or a canoe when it was necessary to tag them fQr identification, or to observe, measure, or manipulate their contents. Many of the waterways in the large northeast heronry of Biven's Arm were kept open by the activity of alligators. Other paths clogged with cut-grass had to be cut open and then frequently maintained. Nests on -iomar Island were reached by walking.

Solar measurements were mare with a dome solarimeter (model no. 615 from Science Associates) which integrated over a wavelength range of 0.3 ; to 3.5 P (ultraviolet to far infrared) and which had s sensitivity of 25.8 m\/Langley (I Langley = ] cal/cO). The solarimete7 itself was mounted on a 5 foot aluminum pole bent at ;5*. At the opposite end of the pole a small bubble compass was attached at 75' so that the bubbe was centered when the solarimeter was horizontal Measurements wera taken only between 1000 and 14,00











hours Eastern Standard Time, when the sur was highest in the sky. Measurements were made of nests at a variety of heights.

Several kinds of air temperatures were taken. Shade and sun

temperatures were usually taken with a Yeliow Springs Instrument (YSI) telethermometer having 10C divisions and using a YSI Type 402 small animal probe. Occasionally, these temperatures were taken with a mercury thermometer having P0C divisions. In either case, the wind was shilded from the device. Black body temperatures were measured using a thermometer whose bulb was painted with flat black paint arid dipped in black soot before drying. The black bulb thermometer, which had 10C divisions, was wind-shielded by a tube painted flat black and having a slot for exposure to the sun's rays. Black body readings were always achieved by holding the thermometer normal to the sun's

radiation.

Body temperatures of nestli ngs were measured using a YS I probe Type 4102. All body temperatures were cloacal and probe penetration dc-ph was 1-3 cm depending on the size of the chick. Occasionally skin temperatures were taken using a YSI banjo probe Type 409. if a parent had to be disturbed from a nest in order to reach the

Penlngstemperature could usually be measured within 5 minutes of adult departure.

N'estlings were usually weighed whenever possible to determine growth characteristics. The weight of small nestlIings was taken with a 100 g capacity Pesola spring sc-ale having I g divisions; the scale had an accuracy of + 1 g. Larger nestlings were weighed on a 200 capac ity double bean balance wi th Q. 1 g div is ions. Weights greater










than 200 g could be measured by putting some handy object on the second pan of the scale.

Egg temperatures were monitored in the nest by pl< Q-a

synthetic egg in the nest. The synthetic egg was made frc a heron's egg shell filled with Dow-Corning medical Silastic 382 Elastomar (Calder, 1971). A thermocouple wire was implanted in the egg which was put in the nest with the natural eggs. The thermocouple lead was kept on a makeshift spool and could be unwound about 100 feet to connect to a portable thermocouple meter (Minimite by Thermo Electric). The parent could be watched safely from a distance while egg temperature was simultaneously monitored.

Because the chicks of all four species studied hatch asynchronously (see Chapter 5), size differences were indicative of relative ages. Thus, it was unnecessary to mark individual chicks. Nests were monitored almost daily after about the twentieth day of incubation to record hatching dates. Nestlings on their day of hatching were considered

0 days old.


Laboratory Techniques

Feather reflectances were measured on a Bausch and Lomb

Spectronic 20 spectrophotometer using a color analyzer reflectance

attachment. A block of magnesium carbonate was used as a white rfle.ctance standard. Reflectance was measured in feathers of intact birds, which had been preserved In a freezer, over a 340 mi-300 mll range.

CoolAing curve's w, e used .- determine the time when horeothermy was fi'.t establi shedi in :.e young of these species. On, nesti Ing






i0



from a given nest would be removed and brought to the laboratory only

1.5 miles away. Usually the bird was returned to the nest the same day, often within 4 or 5 hours of its removal. On a few occasions, heavy rain or darkness prevented this, but the bird would then be returned shortly after dawn the following morning, A thermocouple whose junction was encased in latex was inserted cloacally 1.5-3 cm depending on the nestling's size, and the wire was taped to the pygostyle region. The thermocouple was connected to a Honeywell chart recorder via a thermocouple reference junction. The nestling was put into a chamber which was placed in a water bath at 400C. When the nestling began sustained large amplitude gular flutter (body temperature usually was within one degree of 40'C), it was put in another chamber which had been kept in a 17C water bath. Ambient temperature was monitored with a YSI probe Type 402 which was suspended in the chamber. Cooling was continued at least 60 min or until body temperature closely approximatco mbio'. temperature.

Metabolism was measured in adult heronn using a Beckman G-2 par magnetic oxygen analyzer coinectec n openr circuit (Depocas and Hart, 1957). Air flow of about 1400 dm!I~nn was pulled through the chamber and measured by a Brooks precision rotamter calibrated with a 500 cc Brooks volume-meter after carbon 1ioA de and water were removed. Carbon dioxide was removed from the air with color indicator grade soda lime (8--14 mesh), and water was removed by color indicator regenerated silica gel (grade H, type IV, 6-16 mesh). Only about 200 cm/min was put into the analyzer. Temperature and barometric pressure were recorded, and oxygen consumption was calculated at STP. Body tempera-





II


tures of the herons were taken with a Schultheis thermometer inserted 3-5 cm into the cloaca, within about a minute of the final metabolic measurement. All the adult herons measured, except the Cattle Egret, were raised in captivity and weighed about 20 percent less than wild herons. A weight difference between wild and captive--reared birds has also been noted for the Redhead (Aythya arnericana) by Weller (1957).


Statistical Methods

A variety of statistical tests was used, but all measures of variance given in the body of this paper are standard errors of the means, unless otherwise noted.














CHAPTER 3

NEST SITES AND SOLAR RADIATION


The nest sites of herons have been investigated before, but

usually with respect to height, vegetation type, or relative placement when in a mixed species heronry (cf. Bent, 1926 and Palmer, 1962). Because solar radiation at the nest had never been measured before this study, a confused picture of rest site selection in herons existed.

At Blven's Arm, where most of this study was done, the same gcnerai nesting pattern was seen every breeding season from 1972 through 1975. Louisiana Herons, which arrived first, tended to nest close to open water. Little Blue Herons, in those years when they were present in relatively abundant numbers, tended to aggregate in groups of three to six or more nests, somewhat away from open water. Cattle Eg~rets, which were the last to nest in large numbers, chose nest sites without any obvious reference to open water. Snowy Egrets also appeared catholic in their selection of nest sites, but too few: nested at Biven's Arm to make c, definitive statement. All four herons nested in red maple or elder, the

two most commoni trees, from 1.5-3 m above the ground or water.

Unfortunately, the internal consin-ncy at Bi ven 's Arm from year to year sheds no lig ht on the oh crvad nesting habhits of these herons elsewhere. For example, Bent (1926) forn6 Little Blue Heron nests on the outskirts of W&Yo islands, only 2-4 feet above the ground. On the other hand, Meane (v 955) noted tha t Licttle Bl ue Herons nests 12









were closer to land (farther from open water), mainly in buttonbush swamp, and averaged 8 feet above the ground. Palmer (1962) states that Little Blue Heron nests tend to be grouped apart in mixed species rookeries. Bent (1926) found Louisiana Herons nesting on the ground in one rookery and as high as 15 feet in another. He noted that these herons occupied the center portion of mixed species heronries. However, Palmer (!963) says Louisiana Herons may also group around the periphery of mixed species colonies. Bent (1926) discovered Snowy Egrets nesting in open areas on the interior parts of an island (away from open water) whereas Meanley (1955) found them next to open water in a lake colony. Palmer (1962) suggested that Snowy Egrets can nest over 30 feet above ground, although 5--10 feet was more common. The influence of insolation may not clarify this picture entirely, but it is an important step in this direction.

Solar radiation was measured at nests on Biven's Arm and Riomar Island. Table I shows the solar radiation incident at the nests of all four species of herons; an F test showed that there were significant differences among the herons (p < 0.05). Observations in the field suggested that Louisiana Herons nested selectively in the shade. The other herons appeared to nest without any real reference to insolation. The data in Table I corroborate this: Louisiana Heron nests receive the lowest mean solar radiation input. Furthermore, botn the upper and lover limits of the Louisiana Heron's range of insoiat'c- re lower thcn those of the other three species. Nonethelcs :hre is a broad overlap in the

-anyes of solar input at th- ne~t:s of all four species. In order to c earv show the Cdf-fcrence between Louisiana rnz and the others,










































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the proportion of highly exposed nests is compared in Table 1. The
2
value of 1.200 Langleys/min (= cal/cm .min) is chosen for strictly heuristic reasons as a cut-off in the sun-shade continuum, Nests receiving 1.200 Langleys or more are considered to be under intense insolation. Only 6 percent of the Louisiana Herons fall into this category, whereas at least 20 percent of all the other species do. An a prior. comparison of solar radiation at nests between Louisiana Herons and all the other herons yielded an FvaIue (SokaI and Rohif, 156y) which was highly significant (P << 0o01).

This difference in nest sites is important in Florida because all these herons nest in the spring when average involation is greatest, and birds at exposed nests are subject to great thermal stress. By the time the summer rains begin, much of the nesting season is over. The eventual fate of most Louisiana Heron nests which were found in unshaded areas is unknown. However, one Louisiana Heron nest completely exposed to the sun was watched in 0D74 and is described in Chapter 5. Bent (1926) implies, but does not actually state, that large numbers of Louisiana Herons may nest in the sun. The high energetic price that would be paid for this behavior by the adults is discussed in Chapter 4; the possible affect on nesting success is referred to in Chapter 5.

The orientation of nests in reference to an environmental parameter is well known. Pion Jays (Gymnorhinus ayanocephalus) nest on the south side of trees in order to utilize the heat from solar radiation (Balda and Batman 1572). Desert Larks (Armmoman'es doserti deserti) in the Negev desert build nests facing north and sheltered from the mid-day sun by rocks or vegetation (Orr, 1570). Caliope Hummingbirds (Stll~a ca~liape) prevent nightt i. me heat loss by radiation Lo the heat sink of the








sky by building their nests just below a large branch (Calder, 1971). Roadrunners (Geococcyzx calif nian) often build nests which are partly exposed to the sun but which have bands of shade to provide nestlings with a choice of thermal environments (Ohmart, 1973). Cactus Wrens (Campylorhynchue brunneicapiZus) build closed nests whose entrances face away from the wind in the early cooler part of the breeding season but face them during the hot part of the season (Ricklefs and Hainsworth, 1969).

The effect of solar radiation on an animal can be described by the

term SoA(1 r) [modified from Gates (1962] where So is solar radiation, A is the animal's surface area, and r is its reflectance. Because the Louisiana Heron, Snowy Egret, Cattle Egret, and Little Blue Heron are all about the same size, A can be considered the same for all of them. So is also the same, because they all live together. So any differences in the effects of solir radiation on these birds will be a consequence of their reflectances. Figure I shows the percent reflectances for all these species as a function of wavelength as measured at the dorsal surface. The Little

Blue Heron and Louisiana Heron are markedly darker than the Snowy Egret a d Cattie Eqret. The reflectance curves for the Louisiana Heron and Snowy Egret are probably sli'h.tIy lower than normal because the specimens used had a duller pjluma!ge than is normally observed in the breeding season. However, it is doubtfu1 that the curves would change very much with brighter plumage. Snowy Egrets and Cattle Egrets show a reflectance peak at the long visible wavelengths. Interesting, the Little Blue Heron white juvena plumage reflects more radiation over most wavelengths, including ul!tsviolet nd nea r-infrared. Whether this is characteristic of juvenal piumages in the white Snowy Egret and Cattle Egret is unknown. Refiectance was rot measured for the juvenal plurage of Louisiana Herons,



















































































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but: being dark, its reflectance curve is probably close to that of the adult.

The differences in reflectance mean that a Little Blue Heron or

Louisiana Heron would reflect only about 6 percent of the solar radation striking it, whereas the wh'~e birds would reflect 80-85 percent of the incident insolation. Feathers probably do not transmit much light. But whether the nonreflected radiation (I P) is absorbed by the feathers or~ transmitted directly to the skin, dark birds in direct sunlight must be under a considerably increased thermal load, Louisiana Herons have apparently adopted a strategy of nesting in the shade to cope with this problem. Little Blue Herons have apparently developed physiological and behavioral adaptations which allow it to nest in the

sun. Their strategies will be discussed in Chapters 4f and 5.














CHAPTER 4

ADULT METABOLISM


The energe.ics of temperature regulation in endotherms can be expressed as

M = C(Yb -a) )
a k

where M is metabolism and represents the heat produced, C is thermal conductance, and Tb and Ta are body temperature and ambient Lemperature, respectively. The term C(Tb Ta ) is an estimate of heat loss at low to 0a
moderate T2 when evaporative heat loss is low. As such, C should be considered "wet" thermal conductance (McNab, 1974) since it incorporates a small evaporative heat loss element. At higher T M = C'(Tb Ta + (2)

where L is the latent heat of vaporization, E is the amount of water lost via evaporatioc, and C' is "true" thermal conductance. ;ecause avaporative heat loss was not measured in this study and C is given as a minimum value at only low to moderate T a, equation (1) will be used to Discuss the energetics of thermoregulat ion. In an endotherm's thermal neutral zone, metabolism is basal (Mb) and equation (1) becomes ,0- (3)
bC (b 0
where To is the lower limit of thermoneutrality,

An endotherm has then three variables to effect thermoregulation: M', C, and (T ). Any two of these factors can be modified, thus setting the third. Usually endotherms nodify Mb arnd C (McNab, 1966a,



20







21

970 i ty, A

h,-;- k I the z Dne of +L rmoneun

:onsid-ration of t _;t Jetari-,'iination of M T ) w! 11 follow later.

Mth and conductance a re weight-dependent functions.

Lasic- %Iskl and Dav scin 111967) showed 1 1 of --onpasserines to have the 'b
following rela, ionship

M b = 78. 3TV-1 72 1 (4)

where 2.1 is in !,cal/day and W is weight in kc;. The form of equation (4)
b
can be, modified (McNab, 1974) to ,j-1w = ',.6w (5)
D
'A' h 'a re V I'W cco /9-hr, as -,,jming 4.8 cal/cco (Brody,
15 Irl 1945), and
2 2
W, is in 9. Conductance io birds is related to weight (Lasiewski et aZ., 11)67) as

-/W U.5)

where C 1161 is In ccO,/L' -hrc.

Fiqp,,r, s 2-5 show Ghe of Louisiana Herons, Snowy Eaz-cts,

Cai--tla and Little blue Herons, respectively, as a function of

tep).-)uratoure. I'linimum conductaricc is shown extrapolated thrOUgh

t J! 0 'icbIF; 2 summarizes these. results. Table 3 compares tl)-..:.
b
cus'ei-va'i ba'-"al Fretabr.1 ism and minimum ccinductance with that expected

-ased c l equations (5) amd (6) resp -cLively.


Metabo

r -sti ng ;-"Ietabol i sm was me sured. duri ng da,,, i I 7,ht hou s fc adul ts )F al I 17"our spE ,:les o' herons. Dayt;r e jr are a

"-- "i7ate of energetics 7n the fiel-' when the birds

--s. fe-v nighttir-io measLwerrents of meta ,ol sm












23


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32



in one Cattle Egret (CE1) showed conductance unchanged, but Mb was 24 percent lower than in the daytime. This is in close agreement with the estimate of Aschoff and Pohl (1970) that nighttime Mb is 25 percent lower than daytime Nb. There will be no discussion here of whether it is in fact proper to use the term "basal" metabolism in daytime measurements of diurnal birds. This distinction between daytime and nighttime measurements of metabolism is clouded because 1) the time of day is not always reported and 2) there is a tendency to avoid dealing with the distinction (of. Lasiewski and Dawson, 1967). It should be noted, however, that the low values of Mb reported in this paper as would have been even lower had they been measured at night.

Table 3 shows that db for the smaller individual of a species pair is always significantly highe:- t:h rn for the larger individual (P < 0.05) using Student's t distribution (Simpson et ai.., 1960). Thi5 should be expected from the weight relai.ionship in eqLation (5).

Table 3 also shows that the Louisiana Herons were the only species whose Mb exceeded that predicted by equation (5) (x = 107 percent of expected). Both Louisiana Herons seemed more active or tensers" in the metabolic chamber than any of the other birds. Thus, it is possible that observed metabolism is a slight overestimate. Nevertheless, this species still appears to have the highest '.b Louisiana Herons, which almost always nest in the shade, can be compared to the three other species which nest without reference to the sun. The average Sncwy

Egret M b was 85 percent of expected. Although the range was large (77 and 93 percent t), the average was quite close to the 85.5 percent average of the Cart l Egrets (whose ran e was small). Both of these





33



species are white or nearly whire. Little Blue Herons had an average vb of 66 percent of expected and have dark plumage. A few measurements on a yearling Little Blue Heron with white plumage were- equally I ow.

It is unlikely that differences in Pb are phylogenetic; all the

species except the Cattle Egret are congeneric. Furthermore, as discussed earlier, all have similar distrihutions. The difer nces in N" may be directly attributable to thermal adaptations. Louisiana Herons, by nesting in the shade encountr liow solar heat loads. Snowvy Egrets and Cattle Egrets may nest in the sun but reflect a substantial amount of the sun's visible radiation. What extra heat load they bear is at least partly compensated for by a lower M. The most interesting case is the Little Blue Heron. This species may nest in the sun and, being dark, increase its thermal load. This increased thermal load at Ia To (most of the day ,and most days of the nesting season) probably 'a "..... .

reverses the thermal gradient from skin surface to feather surface, so that net eat flow is into the body (Lustick C- ~a., 1970). The greatly reduced ML in Littie Blue Herons is probably an adaptation to nesting in an environment with high insolation. This is not in itself unique and has been reported for Poor-wil is (Pha" aenopti,-ua -nruttar7l1i) by Bartholomew et a l. (i962)..

The interrelationship between dark color and metabolism has

been investigated before, but generally in cooler environments. It can be summarized as follows: 1) birds dyed black (Hamilton and Heppner, ,167) and dark birds (Lustick, 1969) show a 232 percent reductio n the rate of metabolism when they are exposed to (artific! l) solar radiation at Emnperatures below thermoneutrality.





34



2) This metabolic economy results from a decrease in the thermal gradient from the skin surface to the feather surface, thus lowering conductance (first suggested by Cowles, 1967; demonstrated by Lustick, 1969; Heppner, 1970; and Lustick et al., 1970)., 3) This reduction in conductance may result in a lowered T and probably a lowered upper limit of thermoneutrality (Lustick, 1969). 4) All of this depends on an increased absorption of energy in the visible (Lustick, 1969; and Heppner, 1970) and perhaps near infrared spectrum (Lustick, 1969) by dark as opposed to.white feathers. 5) it is suggested that dark birds stay out of the direct sunlight at Ta > (Lustick eta at., 1970) due to the earlier mentioned reversed thermal gradient between skin and feathers to which they would be subjected.

This last point is of particular interest because Little Blue Herons often nest in the sun at ambient temperatures well above their lower limit of thermoneutrality. It is not unusual for T to exceed
a

35C in the field, whereas T is 27.50C (see Figure 5). Furthermore,
2
incident solar radiation can easily exceed 0.9 cal/cm min (Lustick,
2
1969) or even 1.23 cal/cm2 min (Heppner, 1970). Solar radiation measured at nests (including those of Little Blue Herons) often exceeded

1.30 cal/cm 2min. On May 26, 1973,a maximum of 1.43 cal/cm mrnin was recorded at 1315 hours on Biven7s Arm.

Very few birds having dark feathers or skin and inhabiting hot

environments have been investigated. Two of those may have been hypothermic and may have absorbed the sun nation to raise Tb. Heath (1962) and Obmart and Lasiewski (1971) suggested this for Turkey Vutures (Cathartes aura) and Roadrunners (Geococuyx calZfor~nianus) respectively Road-






35



runners do niot have dark plumage, but their fea-thers may be erected to expose their dark skin when basking. Ohinart and Lasiewsk! showed that although solar radiation Was used to raise T~ in hypothermic Road'b

runners, it was used to lower Tkby reducing conductance in riorrothermic birds (much as in the birds studied by Hamilton and iHeppner and by Lustick) By comparison, Li ttle Blue Herons wiere never observed to bask in the sun (that is no special basking posture or orientation was noted), aria there is no indication of hypothermia in this species.

Table 4 shows metabolic rate as a function of the expected rate from equationi (5) for several dark nonpasserincs which live i v arm, mostly tropical climates; included is the Roadrunner whiich util ize5 its dark skin for basking. If the da-ca of Ohmart arid Las "ewsk 7 (1971) are more accurate than those of 'alder andSmd-i se i9 )

a l ow JMjb is invariably correlated in dark birds with hot environments. McNab (1569, 1974) has noted a correlation between Unpredictable foo0d sources and lowy 11- Both Turkey VJultures (Heath, !Q9621 and Roadrunners (Olbmart and Lasicwski 1971; MOhmart, 1973) have unprediciable food supplies s, as probably do Black Vultures; Littie Blue iHerons (Jenni, 1961-; anid others) and probably/ Manificient Frigatebirds do ot. FHowe'ier, Roadrunners (Ohmart, 1973) M~agnif icient Frigatu;:birds (pers. osand Lifttle Blue

herns llmaynet in Eh ,un. Vul tures do riot n,,.at 1n the sunl but

rinay fO~d in Fifl Isun (pers. obs.). So while food hahbi s iiay contrbute to a l ow Alj. i n dar k b ird,3 i n hoc)t erniv ron ieiit s (r> 7) Inti rr i t
b ~~~~~~a 0 I hsrgr ,

may bie. noted that the temeo roi-eaie Ccwb I d, which) can Jt I ze

S Unq cht Ir)ov c- ir metabol ic output st T < J LutiTkI~ does not have-r a
a .








36


























-0 3

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Ln c
ru 5



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cc,



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C) D

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60


CO

j
'4


3


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37




low Kb although it is very dark. Lasiewski and Dawson (1967) found its Mb to be 99.3 percent of that expected for apasserine of its weight.


Metabolism and Insolation

In Turkey Vultures and Roadrunners, basking makes hypothermic birds normothermic. But Little Blue Herons, Louisiana Herons, Snowy Egrets, and Cattle Egrets all have the same high Tb of about 40'C. Insolation at high T can only make these birds hyperthermic;
a

their Tb rises as they store heat. But because lethal Tb for most birds is around 45-46C, there exists only a small margin for error in hyperthermic birds. The rate of change in body temperature (ATb/At) of a bird which is receiving solar radiation in a hot environment will be a function of internal heat production and external heat. The external heat load will be transmitted through the thermal gradient between feather surface and skin. This can be expressed as conduction

dQ dT
at- dxAy (7)

where Q is heat; t is time; A is surface area; T is temperature; X is distance, and K is a constant. The distance between feather surface and skin may be considered constant, and equation (7) becomes d = C (T -T) (8)
dt 1 f

where Tf is feather surface temperature; Ts is skin temperature, and C1 is a constant K4/X. Therefore, the rate of change in Tb can be expressed

ATb M + C 1 (T f T )
,_ i S (9)
At KW





38



where M is rate of metabolism, K is specific heat, and W is weight.

The net heat gain to the bird at high T2a is

C(T T ) = SoA( r) C2(T Ta) (i0)
f s-tra

where C T Ts) is the conductive heat gain from the feathers, SoA(1 r) is the solar radiation striking the feathers, and C2 (T Ta) is the convective heat loss from the feathers. C2 is a combined constant

(-) for convective heat loss (all other symbols are as previously defined). Equation (10) can be rearranged so that SoA(1 r) + CIT + CT
T 1 2 a (l f C1 + C2 (11)

Substituting equation (11) into equation (9) for T., SoA(1 r) + C T + C T
M + C1 T
AT C + C s
b 1 2 (2
At KW (12)

Assuming T and Ta to be constants, equation (12) simplifies to


ATb = M + K1[SoA( r)] + K2 (13)
de KW
2
C1 C1 T + CIC T
where -. = and Ko = - C.T
S+ C2 1 2 .

It is now apparent that the rate of change of T is a function of both rate of metabolism and solar input. Figure 6 shows the relation between expressed as a percent of that expected by weiht (se Tbie 31 and solar in put. SoA(1 r). YoA(1 was calculated usinC thc



















































































Ln





E V,





40












o

_0
o Z
=3 -C 0 rr Z Z LLJ
0 0 0 : cr
Lli LLJ LLJ
LLJ 7- M :D

Lj < < no
LLJ Z Z 0
L.Li < <


z < 0 0 V) U -i -i X 0

t




-0
C\l

0
U)









-0













0
0
CC) (D

Vq
0










average reflectance from Figure 1, So = 1.43 cal/cm2.min, and A = WO6I7/2 (because rarely would solar radiation impinge on more than 50 percent of a bird's surface area). Figure 6 shows that Louisiana Herons in the sun have high % Mb and high SoA( r) due to their low r. They thus would heat rapidly, and consequently remain mostly in the shade. Snowy Egrets and Cattle Egrets have a slightly depressed % V but their SoA(1 r) is very small due to their high P. Little Blue Herons have the lowest % Mb which compensate. for their high SoA(I r) that a low r gives them. Little Blue Herons can take advantage of a low M. when in the sun so that their rate of
b
T. increase is reIatively slow.
O

Conductance

Table 2 summarizes the data provided in, Figures 2-5 for minimum thermal conductance. Table 3 compares these minimum conductances with the conductances predicted by weight in equation (s). C is equal to or greater than expected in every case. There is a clear trend in these values similar to the data for M,. Louisiana Herons have the
D
highest '7 (129 percent) and nest in the shade. Snowy Egrets and Cattle Egrets which may nest in the sun but are white have intermediate values of 110 and 107.5 percent respectively. The dark, often sun-nesting Little Blue Herons show a C of 98 percent. Dawson and r-Hudson (1970) indicate that birds in hot environments tendc to have high values of C for heat dissipation. Yarbrough (t971 cites data to that effect.

What then is the significance of a reiativeiy low C? it is doubtful that these herons can use it to much ad tae to sulfate themse ves










against the sun's radiation: C is still too high. A low C, however, may have an entirely different function. As a consequence of equation

(3),

Mb/C = T T (14)

That is the zone of thermoneutrality is a function of Mb. and C. A
b
reduction in Mb would lower (Tb T) unless compensated by a lower C. Thus a lower C may be the means by which an endotherm maintains a relatively wide thermoneutral zone even while decreasing its heat production. This may explain the low C value of Little Blue Herons. Even so, its zone (13.4 + 0.70C) is significantly smaller (P < 0.05 using Student's -) than any of the other species (16.2 + 0.70C) when c-iculated as ,/ An analogous situation may be found in Poor-wills. Poor-wi is rinost inr the sun and have a very low (49 percent) Mb (Barthoioomew t c7.~, 1962). They also have a C of 94 percent. This C pro 2b'y is not so important for its insulative properties as it

is in guarant eng a wider thermoneutral zone. for example, the the rmoneutr~l zone of a Poor-will is 6.560C as caelcilated by N, /C. If the Poor-will's C had been 1C0 percent, Mb/C = 6.18'C. Higher C, though good for heat dissipation would narrow further this species' ability to cope with its environment.

Empirically determined (Tb TO) in Little Blue Herons (12.8 +

0.50.) is not significantly different from that of the other herons (14.4 + 0.400C). This is due to the high T (28.00C) of CE 4. An estimate of t.he lowcr 1l Imit of ,thermoneutrality at minimum ther-mal conductance T1, is given in Table 6. Because the changes in C with decreasing





43



: are essentially behavioral (see below), (Tb T ) of Little Blue Herons (12.8 + 0.5C) is significantly lower than that of the other herons (5.9 + 0.5'C) using Student's t-test (P < 0.05). Determination of Thermal Conductance

Conductarnce is the coefficient of heat loss in the Newtonian model of equation (). In an endotherm whose total response to Ta below T is chemical regulation, C is the slope of the regression of metabolism, on decreasing Ta. In these cases C extrapolates to Tb in accordance with equation (1). This is rarely the case in birds, however. Most birds show a mixing o: chemical and physical regulation at T < 2 This a

manifests itself as a steadily decreasing C as Ta decreases (SchmidtNielsen, 1975: 323). in this case the regression of metabolism on T extrapolates to a figure in excess of Tb. The slope of this regression if presented as C will be an underestimate. Contrary to King (1964) this does not invalidate Newton's law of cooling as a model (Scholander at. A., i5O); it simply reaffirms that birds mix physical responses with chemical responses to low T a in these cases there may be a family of
a

slopes which decrease as T decreases and which converge at Tb (ef. Figures 2-4). Minimum thermal conductance is that conductance which represents purey chemical responses to -cr.ased 'Z ; where mixing. of
L

responses occurs, it is found at the lo west T values. This riniui,
a

cnduzza e is...the C of equat ion ( )

.Althogh C cannot be equivalent to the slope of a line which does not extrapmalza to TVit can be calculated in several ways. In this par f ,was cc ndero, to he the lc sE c.e of the fami i of slove-










which converge at IbY C has been calculated based on the metabolic response to the lowest T ameasuyed (Lasiewski et A., 1970). Calder and Schmidt-Nielsen (1967) considered





where S is surface area. This is a form of equation (2). Here is true dry thermal conductance. Unfortunately, most of the literature is based on "wet"l thermal conductance (cf. Lasiewski et A.~, 1967). However, since Ta is normally low, LiE should be fairly small and CY should be only slightly less than C. Using the data Calder and Schmidt--Nielsen (1967) present for the Roadrunner, I calculated the minimum slope C (extrapolated to T b) as 0.0473 cc02 /ghr-0C (100 percent of expected). This differs from the average low T C'Y of
2
1.30 calm .hr-'C V= 0.042 ccO 2/g-hr-0C) by less than 12 percent.

A slightly more accurate calculation of C would involve using the

same data points which produce minimum conductance in a slightly different way. Recognizing that the extrapolation to T involves an average T b each metabolic value can be regressed on its own T b. A family of slopes, whose average is an estimate of C, resultLs. Table 5 shows the difference between "mi niirm" conductance and averageg" conductance; the former is a slope itself, the latter is an average of values determined by several slopes. In seven out of eight cases in Table 5, the average value gives a higher estimate of C. If these results can be generalized, all methods of calculating C di fferently than the last averaging method, tend to underestimate the parameter.





4f5







Table 5. Comparative measurements of conductance
in four species of herons.


Animnal Crin Cavg Cavg/ mi


LOU 2 0.0579 0-0575 0.993

LOU 3 0m602 o.c616 1.023

SE 10.0450 o.0464 1.031

SE 2 m.549 0.0566 1.031

aE 4 0.0574 0.0641 1.117

CE 5 0.0427 0.0431 1.009

LB 1 o.0~48 0.0482 1.076

LB 2 0.0475 0.0553 1.164


acmin C1W in Table 2.









Temperature

it was stated earlier that once M.b and C were established (T T) was set. This is simply a consequence of equation (3). (T T) is of course itself not a single variable but is the difference between two variables, one of which may be selected for. Tb is usually higher for birds than mammals of the same weight. This is a consequence of a generally higher Mb and lower C in birds (tMcNab, 1966b and 1970). Tb and T k are related to each other and body weight. This can be expressed as

T = 5.41FW023 + T (16)
b X,
0 23 -0.28 -01
where 5.41 = 4.6w O2/o.85W"O5 the expected ratio (Mb/C) for

birds (Lasiewski and Dawson, 1967 and Lasiewski et aZ., 1967) and F is the relative ratio between observed and expected M b/C [equation (16) is modified from McNab, 1970]. Table 6 shows that Tb is close to that predicted by equation (16). Tb measured for Cattle Egrets in this study (40.60C) agrees well with the T. for wild and captive Cattle Egrets (40.o46'C) reported by Siegfrie.i 1938) Wet.are (1921) reported Snowy oEgret T at about 400C which corres onds closely to the 40.6'C reported here. Table 6 also shows that the diifferences in (Tb 1) between Little Blue Herons and the ocher three species mentioned earlier is due to differences in T. When Tb of i ttle 3lua Herons (40.3 4 0.20C) is compared to an average Tb for miiana er,)on, Snowy Egrets and Cattl e Egrets (40.5 0.2*C), there is no significant t difference nor is there when com'paring T~. But T for Littl e Be He rons s significantly higher than the ohe: species (27 + 03 v. 4.7 v+ 0I., < 0.05) using a








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48



t-test. it appears that a high Tb is being selected for. In the equation

A =(T, T) T. is the dependent variable. Little Blue Herons then show a different temperature response to hot environments than do Turkey Vultures (Heath, 1962) and Roadruiners (Ohmart and Lasiewski, 1971) both of which bask to compensate for hypothermia. Little Blue Herons, which are normothermic and do not bask, adjust T As stated earlier, the differences probably are due to the unpredictable food supply of Turkey Vultures and Roadrunners, Energetic Implications

Yarbrough (1971) has argued that tropical or desert (i.e., warm

habitat) birds show a low value [his (M,/c) 1 ] and have a low (Tb T) The F values in Table 6 are in fact low with the Little Blue Herons (Q = 0.674 + 0.040) being significantly lower than the pooled F of the other species (0.803 + 0.034) using a t-test (P < 0.05). Table 6 also shows that (T I,) is lower than the expected ratio (Mb/)e would predict.

Siegfried (1369) measured "aviary-existence energy level" as

100 kcal/day for a 383 g Cattle Egret at 16-19C. This is equivalent to 2.27 ccO,/q'hr where averageT = 7N..C.. Siegfried showed that this was about triple standarc metabolism NJb,; acruaily it is 2.61 times IN [caculace as predicted by Lasiewski and Dawsor (0367) in equation ,5) shovel. Figure A shows that a Cattle Egret uses an average of .21 cc~o/,hr at 17.5C. Hence aviary existence energy is 1.88 times resting metabolism ate 16-19C range, This value is a more useful estimate tha:n S;E.ured'. If one could generalize to all ambient





49



temperatures, existence energy is about double resting rates, a figure which could be used as a starting point for predicting energy budgets for these birds.














CHAPTER 5

THERMOREGULATION AT THE NEST


If it is true that the relativel-y low metabolism of some adult herons is, at least in part, a consequence of nesting in areas exposed to high solar radiation, then behavioral modifications might also be expected. The preference among Louisiana Herons for shaded nest sites is one such modification. A field study of the nesting habits of Louisiana Herons, Little Blue Herons, Cattle Egrets, and Snowy Egrets with respect to thermoregulation revealed several behavioral differences among these birds.

All four of these species build relatively open stick nests which are unlined. They all lay eggs which are essentially indistinguishable in color (light blue) and size. Varying clutch size in these species has been reported in different years and localities ranging from 3 to 7 eggs (Bent, 1926; Howell, 1932; Meanley, 195.5; Teal, 1965; Dusi, 1966; Blaker, 1969; Hopkins and Murton, 1969; Dusi and Dusi, 1970; Shanholtzer et al., 1970; Weber, 1975), but all showed a clutch size of about 3 in the Biven's Arm study area (Table 7). The young of all four species hatch asynchronously after similar incubation periods of 23 + 2 days (Palmer, 1962; Weber, 1975).

One conspicuous difference, however, is in nestling plumage

coloration. The dark Little Blue Heron has young with white plumage as do the white Snowy Egret and Cattle Egret. The relatively dark Louisiana Heron has young with a dark brown plumage. Paradoxically,


50







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52



all four species hatch with only sparse down and heavily pigmented dark blue (occasionally dark green in Cattle Egrets) skin. These differences plus those already discussed in the adults are mainly responsible for the differences in thermoregulatory behavior at the nest.


Incubation

Attentiveness, or time spent on or at the nest is one of the most important parameters of incubation. Unfortunately, this term connotes different behaviors in different kinds of incubators. ''Attentive period'' for single-sex intermittent incubators means the time spent at the nest, whether on the eggs or not, as opposed to ''inattentive period'' spent in foraging, maintenance activities and social interactions. This is the usage of White and Kinney (1974) even though they define attentive period as the ''time spent on the nest per unit time'' (emphasis added). For bisexual incubators where both parents alternate at the nesL the situation is different: one parent is always present at the nest. The attentive period for these birds is best considered as the time spent on the nest, that is actually sitting on eggs. Then inattentive time becomes the time not actually on the eggs; it is spent the same way as in singlesex intermittent incubators except for foraging. This distinction would follow Skutch (1962) although not using his ''session/recess'! terms,

Attentiveness was examined for Little Blue Herons and Cattle Egrets to assess the effect of plumage color on the incubating adults. The dark Little Blue Herons and the nearly all white Cattle Egrets were selected as representatives of the heron color spectrum. Nests in the shade were compared to nests in the sun; the latter were inspected on sunny and





53



cloudy days. To best handle the data, an arbitrary decision was made to

consider all nests more than 50 percent in the sun as "sunny" and nests with 50 percent or less sun (due to clouds or natural shade) as ''shady." A total of 35.9 hours were spent watching seven Little Blue Heron nests and four Cattle Egret nests for 60 min or longer periods at randomly selected times between 1030 and 1730 hours under a variety of environmental conditions. Attentiveness (percent time on the eggs) was considered as a function of ambient temperature. The results are presented in Figure 7. As T a increased, attentiveness tended to decrease for both species. Shady nests were about the same for both, although Cattle Egrets appeared to be slightly more attentive. At sunny nests, however, although attentiveness continued to f ll with increasing T a Little Blue Herons spent noticeably less time on the nest than did Cattle Egrets. Lines connecting two points in Figure 7 represent pairs of measurements made simultaneously. Here environmental conditions such as wind, humidity, and cloud cover were identical; only the degree of cover from the sun varied. All pairs involved one sunny nest and one shady nest. Four of five lines connecting pairs of Cattle Egret nests are mostly horizontal reflecting the expected difference in T a between a sunny and shady nest. However, the lines connecting pairs of Little Blue Heron nests have a considerable vertical component as well. Little Blue Herons were behaving in the sun as if T a were even higher than it was. This must be a consequence of dark plumage increasing the thermal load on a bi'rd, probably by reversing its skin to feather surface thermal gradient (see Chapter 4). The general relationship of decreasing attentiveness with increasing Ta has been noted before for Cattle Egrets (Lancaster, 1970), White Ibis


































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56



(Rudegeair, 1975), and for single-sex incubators as diverse as Mallards (Caldwell and Cornwell 1973) and small passerines (White and Kinney, 1974~). This relationship is probably directly related to incubation. As Ta approaches egg temperature, the gradient between them is reduced, and longer periods of inattentiveness can be tolerated without risk of a critical drop in egg temperature. For Little Blue Herons on shady nests and all Cattle Egrets, time spent off the eggs was used, in decreasing order of occurrence, to preen, adjust nest material or eggs, engage in aggressive interactions with neighbors or interlopers, and gather new nest material. All these activities but the last, which was very rare, occur at or very close to the nest site.- The most common behavior of Little Blue Herons on sunny nests when off the eggs was to adopt a ''shading"' posture as described by Bartholomew (1966). Here the bird's back is to the sun, wings are drooped, and gular flutter is common. There is no reason, however, to believe that these birds are trying to prevent their eggs from overheating; otherwise Cattle Egrets at the same T aand on sunny nests would show this behavior. More likely, these Little Blue Herons are trying to prevent themselves from overheating by posturally increasing their convective heat loss. Nevertheless, by effecting this posture while standing over the nest, solar overheating of the eggs is prevented. On a few occasions though, a Little Blue Heron would walk off a sunlit nest and stand for a few minutes in the shade. Postural responses to thermal stress in incubating adults have been reported before by Howell and Bartholomew (1962) for Sooty Terns (Sterna fuscata) and by Bartholomew (1966) for Masked Boobies (Sula dactylatra). The thermal stress resulting from insolation on dark plumage was also






57



evidence by the fact that Little Blue Heron adults in the sun always showed gular flutter (a mechanism of evaporative heat loss) at lower Ta than Cattle Egrets, Snowy Egrets, or White Ibises (pers. obs.). Furthermore, Cattle Egrets on sunny nests showed the same postural ''shading'' as Little Blue Herons, but only at more elevated levels of T a

Two Louisiana Heron nests were observed on two occasions. One

adult was on the eggs 96.8 percent of the time where the nest was shaded from a cloudless sky at T a = 28-C. The other adult was on the eggs 100 percent of the time on a very cloudy day when Y' = 28-C also. These
a
observations agreed with my general impression that Louisiana Herons, which usually nest in the shade, are very attentive. However, no Louisiana Herons that nested in the sun were observed (almost none existed), and there are insufficient data to draw general conclusions about attentive time for this species. Snowy Egrets were not observed at all in this regard.

on three occasions, it rained during observations of Little Blue Herons and Cattle Egrets. Attentive time increased to 100 percent each time. This increase in attentiveness during rain is also reported by Caldwell and Cornwell (1975) and Rudegeair (1975).

Egg temperatures were monitored with a synthetic egg. In the

morning, egg temperatures were usually below 34% and rarely fell as low as 20'C. Egg temperatures stabilized by around noon and remained around 35'C regardless of attentiveness by the adult (see Table 8). The stabilized egg temperatures of these herons are relatively low compared to those of most birds studied (Drent, 1973). Lundy (1969) found in chickens that egg temperatures below 35% reduced hatching success. The egg tempera-








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59


ture of the Herring Gull (Larzus argentatus) which like ardeids has bisexual incubation, is about 380C (Drent, 1970).

These apparently low egg temperatures may be the-result of using a synthetic egg. The thermocouple from the egg left through the bottom of the nest. Perhaps because it could not be turned or otherwise moved, the synthetic egg was usually pushed to the bottom of the nest. At least once the egg was pushed nearly completely through the bottom of the nest. An egg pushed down in the nest would not only be farther from the incubating adult and any incoming solar radiation, but also it would be subject to updrafts and winds through the open weave stick nest. This would be exaggerated in the cool morningwhen the birds are moving off the eggs occasionally. Egg temperature can be adjusted by the tightness of sit (White and Kinney, 1974). Tightness of sit can be expected to increase at night when the adult settles down to sleep. One nest, CE 108, was monitored one night from 2300 hours to 0500 hours the following morning. During this period, the adult never got off the eggs and egg temperature stayed within a degree of 350C. This tends to confirm the hypothesis that the morning egg temperatures were low as a result of the synthetic egg being pushed to the bottom of the nest and in the absence of good tightness of sit.

There is no evidence of the presence or absence of incubation patches in Ardeidae in any of the literature searched (e.g., Jones, 1971).


Behavioral Temperature Regulation

Chicks of all four heron species usually hatch asynchronously. Blaker (1969) showed that asynchronous hatching in Cattle Egrets had





,60



a marked effect in nest siblings' competition for food. Rarely waste third (or fourth) nestling able to compete with its older siblings, and if it survived, it showed a slower rate of growth than the first two (Blaker, 1969; Siegfried, 1972). Ohmart (1973) demonstrated that asynchronous hatching in Roadrunners allowed regulation of the brood size in relation to food supply.

After hatching, attentiveness should be defined as time spent at or nearby the nest. The parents continuously attend the young after hatching until the eldest is at least one week old. Four Louisiana Heron nests, 14 Cattle Egret nests, and three Little Blue Heron nests were watched at random hours every day after their eggs hatched for evidence of parental absence. The first parental absence from the nests was observed when the oldest nestling was 12.6 + 0.7 days (range B-19 days), with no apparent differences among the three species. Most of the nests had two nestlings, although only one nestling occurred in one nest of each species. Because observations were not continuous, 12.6 days is probably an overestimate of the eldest nestling's age at first parental absence. Weber (1975) noted continuous attentiveness for about 14 days in Cattle Egrets whereas Blaker (1969) noted that no nest is left ''unguarded'' before chicks are 10 days old. In three nests with twins, Blaker noted continLIOUSadult presence for an average of 13 days, and in three singleton nests he noted continuous attentiveness for an average of 16.7 days. One Louisiana Heron nest is not calculated into the mean nestling age. Its only young was 23 days old at first parental absence; the significance of this nest will be discussed later. The first parental absence does not signify an end to the attentive period. On the contrary, the parents






61



may still spend a great deal of time at the nest, particularly in the middle of the day when T a is maximum. Attentiveness does gradually diminish with time however.

During the period of attentiveness, the parents behave in such a way as to ameliorate the thermal environment. This behavior changes as the nestlings grow older. The adult usually broods the nestlings for about one week after hatching. At first this may serve the additional purpose of incubating any remaining eggs in the nest. But even after the last young is hatched, the adult will frequently brood its ne5tlings. This behavior, which shields nestlings from cool T a or cooling breezes, was usually seen in the"morning or evening hours. Nestlings as old as 5 or,6 days were often brooded in this way. Occasionally older chicks were similarly brooded; for example, a Snowy Egret (SE 3) parent lay on its 13-day-old nestling under a cloudy sky when T a = 320C or less on May 31, 1975. It is possible that this parental behavior was normal at night. On July 7, 1974, at 1600 hours an adult Cattle Egret (CE-106) brooded four young aged 4-7 days; another Cattle Egret (CE 109) was seen to gently lie down on the 12-day-old nestling which had been standing between its legs; a third Cattle Egret (unnumbered) stood hunched over with its large 18 day nestling between its legs.

No evidence of shivering was noted for any of the species as

reported for Cattle Egrets by Blaker (1969) and Hudson et aZ. (1974), primarily because most observations were made from a distance, usually with the aid of binoculars. However, the parental behavior of brooding the very young (< 6 days) nestlings would probably compensate for any lack of the ability to shiver in these birds. Interestingly, Hudson





6k



et cZl. (1974) show that shivering is not effective in Cattle Egrets until they are about 5 days old.

As the nestle ings grew older, parental attentiveness less often took the form of brooding and more often consisted of standing one or two meters away from the nest. During this period the behavior of the nestlings played a major role in determining their thermal state. Young nestle ings were capable of moving about in the nest. Because many nests had at least a partial exposure to the sun during parts of the day, the chicks could often move in and out of the sun by moving only a short distance. This situation is also seen, but somewhat more elaborately, in Roadrunners whose nests are apparently built to provide both sun and shade (Ohmart, 1973). During cool or windy periods when the adult was off the nest, the young herons would huddle together. This behavior was first noted in Cattle Egrets by Blaker (1969) and was observed by me more often in this species than in the other herons.

At high Ta when the adult was off the nest, the nestlings of all four species often responded behaviorally. The nestlings moved apart and spread out in a prone position, sometimes allowing their heads to hang' outside the nest itself. This posturing probably increased their surface/volume ratio and thus increased convective heat loss. As the nestlings grew larger this prone posture gave way to the posture described above for adults as "shading:'' back to sun, wings drooped. The most commonly observed response in nestlings to high Ta was gular fluttering. All four species show the capacity to gular flutter from hatching. This has been reported before for Cattle Egrets by Blaker (1969) and Hudson et al. (1974). Hudson et al. showed that young nestlings could dissipate





63



1.4-2.9 or more times their metabolic heat production by evaporative water loss at high T a (44-450C). Most of this evaporative water loss came from gular fluttering and panting, but some was presumed to come from excrement smeared on the abdomen of young nestlings before their plumage was fully developed. I have observed excrement on the abdomens

of nest] ings of al I the heron species investigated. Whether or not high T stimulates excretion in the nest is speculative. However, one
a
nestling Cattle Egret raised in the lab was capable of excreting outside the ''nest'' after about 10 days. Ricklefs and Hainsworth (1969) showed that Cactus Wrens nesting in the desert left more fecal sacs in the nest as T a increased through the breeding season. This resulted in a lowering of nest temperatures; effects on nestling T b were not discussed.

The interaction of nestling and parental behavior may often be

complex and results, presumably, in the stabilizing of T b at a high level in the nestlings. Two examples should illustrate these behaviors. Two neighboring Cattle Egret nests were observed July 3, 1974 between 1030 and 1200 hours. At the start of this period, the wind was moderate to brisk, and T a < 280 under a cloudy sky. The parent at nest CE-105 lay on two nestlings, aged 3 and 4 days, until about 1100 hours. At that time, the sun broke through, and the wind calmed. T a in the sun was not determined at this time, but when the sun reappeared an hour later under similar conditions, T a = 36-OOC. The parent then stood up and shaded its two nestlings. At 1120, clouds obscured the sun again, and the adult lay down on the nestling once again. In nest CE-106, there were four nestlings which included individuals older than the nestlings of CE-105. The parent of CE-106 was somewhat less attentive, so thermoregulation






64



became largely a function of nestling behavior. The four nestlings in CE-106 were 6, 5, 4, and 3 days; the youngest nestling was a runt. At 1030 hours, the parent was off the nest, and the young birds huddled together. At about 1055 the parent lay on the nestlings. Five minutes later the sun reappeared, and the parent arose, briefly shaded its chicks, then moved about 2 feet from the nest. By 1110, all of the young had spread out assuming a prone position; two,which could be clearly seen, gular fluttered. After the sun disappeared behind clouds at 1120, gular flutter stopped. By 1130, the wind had picked up again and the nestlings huddled together.

By the time heron nestlings are 9 or 10 days old, they are capable of leaving the nest to climb on adjacent branches. These "branchers" (Siegfried, 1966b) were first noted at 9 days old in Cattle Egrets by Blaker (1969). The ability to move out of the nest has great significance for the brancher in altering its thermal environment. By the time they were 12-13 days old, the branchers were so adroit at leaving the nest, they could rarely be caught.

The thermoregulatory effect of parental and then, increasingly, nestling behavior can be seen by looking at nestling body temperatures. Body temperatures of young birds were taken within 5 min of entering the heronry. The results are shown in Table 9. Of the young Louisiana Herons whose I' was measured, three came from singleton nests, 10 from nests with twins, and 10 from nests with triplets. Of the Snowy Egret nestlings, one was a singleton, the other two were twins. Two of the Cattle Egret chicks in Table 9 were singletons, five were from nests with twins, and nine were from nests with triplets. Among the Little








65















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66



Blue nestlings measured for body temperature, only one was a singleton and six were twins; no triplet nestlings were measured. No differences in T b could be attributed to the number of young in the nest, T a, or age in any species, as reflected by the small variance around each mean Tb The very similar mean T b among all the species is probably a consequence of their similar behavioral responses to the thermal environment even where the insolation varied greatly (see Chapter 3). The values of T b reported in Table 9 are about 10C below the 400C reported for Cattle Egret nestlings by Hudson et al (1974). Such a difference could easily be caused by the 5 min period of parental absence due to my disturbance.

The inability of young nestlings to maintain a high T b in the

absence of a parent was evident in the following cases. On May 9, 1974, a Cattle Egret nest (CE 4) in the shade (T a= 300C) had three nestlings, aged 6, 5, and 1 day old, all huddled together. T b was 36.50C, 36.O0C, and 33.00C respectively. The parent had been scared away by my presence an undetermined period of time earlier (>> 5 min). On May 19, 1975, an adult Snowy Egret was flushed from its nest (SE 3). After 15 min, the T b of its singleton nestling was 35.30C. The nestling had hatched that day and weighed only about 18 g. The lower T b of the smaller chicks in these two cases is related to their level of homeothermy as discussed below.


Development of Homeothermy

Dawson and Hudson (1970) and Dunn (1975) review an extensive

literature on the development of homeothermy in birds. The only study of the development of homeothermy in ardeids has been by Hudson et al.





67



(1974) who investigated Cattle Egrets. Their study in fact demonstrated the development of endothermy in Cattle Egrets. That was not possible in this study since metabolism of nestlings was not measured. As a result, homeothermy cannot be linked conclusively to a given level of internal heat production but must instead be defined as the ability of young birds to maintain a T at least 75 percent of adult 7 at low T b 'b a
(ca. 20'C) for one or two hours (Dunn, 1975). This ability may be due to increased insulation because of emerging plumage, increased vasomotor control, reduced surface/volume ratio, better muscular coordination (e.g., more effective shivering), higher rate of metabolism, or a combination of some or all of these.

Fi gures 8-11 show nestling cooling curves at different ages for Louisiana Herons, Snowy Egrets, Cattle Egrets and Little Blue Herons respectively. The temperature differential (T b T a ) is plotted as its natu ral log against time. The resulting slopes are therefore the cooling constants (a) with units of min- 1. Generally, the older the nestling, the slower it cools. Therefore the cooling constants get progressively smaller with increasing age. At least brief periods of homeothermy are attained by 13 days.

Many of the curves for a given individual show two or more slopes.

This is first seen in nestlings 8 or more days old but one Snowy Egret showed it at 6 days. A young heron is apparently capable of maintaining increasing (T b T a ) levels for short periods of time. After about an hour (T b T a ) drops at a rate reminiscent of younger nestlings. This is suggestive of a drop in metabolic activity. When the final cooling constants (i.e., the only cooling constants in the younger nestlings)













































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are compared for most species, they are remarkably similarshowing only a small decrease with age (see Table 10). This residual difference is probably due to a decreasing surface/volume ratio as well as emerging plumage. It is likely then that initial cooling constants in older nestlings reflect a considerable metabolic contribution.

The initial cooling constant is probably of greatest ecological

significance. It represents the cool ing a nestl ing would first undergo when left alone. Usually the change to a higher a does not occur for at least 40 min. By that time, these nestlings (nearly all branchers by this age) could change their thermal environment by, for example, moving into sunlight. Only on cool windy days in the absence of a parent would this time be exceeded before a suitable environment could be found. Ellis and Ross (in prep.) demonstrated that cooling constants could be used to predict the time a poikilotherm (here prehomeothermic birds) had before its TZ' approached Ta:
kn(T b -T a)t (T bT a) i(1)

-a

where (Ta- T ). is the initial temperature differential.

The natural log of the initial cooling constant is plotted

against age in Figure 12 to determine differences among the species. The correlation coefficients, r, for Snowy Egrets, Little Blue Herons, and Cattle Egrets are 0.99, 0.99, and 0.95 respectively, whereas r for Louisiana Herons is only 0.81. The slope (-0.173) for Louisiana Herons is the lowest, and although it is close to that of Little Blue Herons (-0.186), most of the Louisiana Heron data lie above their slope. The Lousiana Heron also showed the poorest level of homeothermy at the 13-14 day age class. This all tends to indicate that Louisiana





77
a.
Table 10. Cooling constants in nestling herons,


Period
Species Age II II

Louisiana Heron 2 59.57
4 14.52
4 19.07
5b 32.04
6 25.79
lOc 16.12
11 16.52
14 2.66 10.79
Snowy Egret 2 23.95
5b 17.20
6 11.20 13.15
9 5.12 10.67 17.80
13 2.77 7.80/6.07 14.80

Cattle Egret 3 23.47
4 23.29
8 7.10 19.24 35.84
9 9.75 17.58
9 6.48 21.30
13 1.20 9.40 18.18
14 2.10 9.52 19.86

Little Blue Heron 3 23.11
5 19.38
10 6.58 10.98 21.87

a Taken from slopes of Figures 8-11; numbers in body of table are x10-3.

bRaised in lab; body weight corresponds to 2-day-old chick.

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80



Herons achieve homeothermy more slowly than the other species. At present, the poor fit of the Louisiana Heron data to their slope cannot be explained. However, it results in such anomalies as the two 4-day-old chicks having a cooling constant comparable to an 11-day-old nestling.

Cattle Egrets show the fastest development of homeothermy, followed by Snowy Egrets. Their slopes from Figure 11 are -0.261 and -0.226 respectively. Hudson et al. (1974) indicate that endothermy is achieved in the 10-12 day-old class of Cattle Egrets.

Two nestlings were removed from their nests at 1 day and taken to the laboratory. One was a Louisiana Heron, the other a Snowy Egret. Growth was stunted in these birds so their weight at 5 days, when cooling curves were maderesembled that of nestlings only 2 days old. Nevertheless, the Snowy Egret showed a cooling constant in keeping with its age. The 5-day-old Louisiana Heron is difficult to evaluate due to the scatter of data in that age group.

The ability of a brood to become homeothermic before any individual in it could regulate has been reviewed by Dunn (1975). An extreme case was cited by Yarbrough (1970) where a brood of 4-5 Gray-crowned Rosy Finches (Leucosticte tephrocotis griseonucha) could regulate their temperature 6 days before any individual could. The huddling in herons as described above is probably a mechanism to allow collective homeothermy at relatively low T (< 300C) before the individual nestlings were able
a

to thermoregulate. As might be expected, older nestlings (> 10 days) were not observed huddling. Unfortunately, cooling curves were not developed for nestling broods in this study. But McManus and Singer (1975) working with the Mongolian gerbil (Meriones unguiculatus)






81,



demonstrated that thermal conductance fell as brood size increased. Huddling probably acts only in reducing heat loss by decreasing the effective surface/volume ratio.

Dunn (1975) in a review found that growth rate is the best predictor of the age of endothermy (although many of her sources provide age of homeothermy, not true endothermy). Growth in all four species of herons is given in Table 11. These values were converted to growth constants (K) using the method provided by Ricklefs (1967). The three Egretta species seem to fit the logistic equation best. The Cattle Egret shows a slowing of growth in the later ages measured, which is a characteristic of the Gompertz equation (Ricklefs, 1967). Nonetheless, the Cattle Egret fits the logistic equation slightly better. This is interesting because the Gray Heron (Ardea cinerea) and the Green Heron (Butorides virescens) fit the Gompertz equation (Ricklefs, 1968). T'he data provided in Table 11, especially for the Little Blue Heron and Snowy Egret, cover too small an age range to claim one equation with certainty. However, unless later growth is very different, all species do seem to fit only the logistic model. The asymptotic weight in the Ricklefs model could not be determined by strictly graphic methods, due to the lack of growth data for older nestlings. Instead an asymptote was selected to give the highest correlation coefficient (r) when age was plotted against the logistic conversion factors. The asymptotes are presented with the growth constants in Table 12. The Little Blue Heron (430 g) and Louisiana Heron (420 g) asymptotes probably do approximate the upper limits of adult weight. However, 530 g is certainly an overestimate for the Snowy Egret. Even so, if the Snowy Egret had had an





82


Table II. Growth in nestling herons.a, b

Age Louisiana Heron Snowy Egret Cattle Egret Little Blue Heron

0 19.4 + 0.4 21.5 + 2.0 19.7 + 0.7 22.0 + 0.6
(17) (T) (8) (3)

1 25.4 + 0.6 26.5 25.5 + 0.3 23.8 + 1.2
(IT-) (1) (IT) (2)

2 35.3 + 2.2 33.0 35.1 + 1.7 30.0
( ) (1) (IT) (1)

3 44.2 + 3.1 -- 42.1 + 3.8 39.0
(T) (T) (1)

4 60.0 + 5.2 71.5 +14.5 55.8 + 1.8 -( T (7) ( )

5 77.7 + 3.8 62.5 69.5 + 2.8 78.8 + 5.7
(9-) (1) (7 ()

6 -- 126.5 81.6 + 4.3
(1) (4)
7 109.1 + 15.4


8 -9 173.0 135.5 + 7.4
(1) (3)

10 211.5 151.5
(1) (1)

11 182.0
(1)

12 206.0
(1)

13 178.2 + 7.2
(27

14 181.0
(1)

aMean s.e., measured in g. bNumber in parentheses is sample size.






83


asymptote of 420 g, K would increase only 3 percent from 0.312 to

0.322. The asymptote for the Cattle Egret is certainly an underestimate of adult weight in the field. Hudson et al. (1974) equated their asymptote with fledging weight, but I see no a priori reason for doing this. The asymptotic weights for Cattle Egrets measured in Florida in this study agree with those provided by Hudson et aZ. for Texas and South Africa (based on data from Siegfried, 1972).

Table 12 also shows the rates of development of homeothermy (H) which are the slopes from Figure 12. Cattle Egrets in Florida (and Texas) grow more slowly than the other species, and develop homeothermy more rapidly. But this inverse relationship does not seem to hold among the other herons. Over a wider range 'of species, Dunn (1975) found that a positive relationship existed between growth rate and the development of homeothermy.


.Color in Nestling Herons

In very small, essentially ectothermic chicks, heat loss is a major problem. As the bird grows, its surface/volume ratio decreases, its metabolic rate increases, and its insulative plumage increases. At a certain point, heat loss is no longer the main problem, but rather heat gain becomes of primary concern. At this stage, metabolic heat production may be too high, surface/volume ratio too small, and insulation too great to withstand long periods of solar radiation. Usually all of these problems are controlled by parental or nestling behavior. But occasionally, behavior will not suffice.

One way of coping with heat loss for short periods is solar

brooding. This has been reported for Roadrunners (Ohmart, 1973) and








84









r- 0

Q) u
S- >.- > -, -W crl
71 _0 -0 -a -0 r-- r- ,
0 :3 :3 :1 :0 j ?11
V) 4 -W 4 -W
cn cn Ln m -k
)) C ) 0
4J 4-J -W -W
C: C:
Q) 0 0 0
M LO (n Ln Ln Ln 0
Q) a) q) 0 -a o m
0 L- L- S- L- :0 :3 -0
CL 0- 0- Ln





u
r co
C"A f
I
C C:* 0* C;
CT) f
0




00 00 C,4 \ D
U) u C) co -zzr m N
m cli C14 m

CD 0 C) CD 0 C:)
D


0
L
(1) -0
-C (1)
4J
C: 0
4 0 C:) (D C C) C)
0- m m m r-I CD
E -zt- -:T- UN CN vq M
4-J >3: un
0 <
L

ru
>
4.J
u
c 4-1
ro 0
c Q)
E 4 ro ru (U Ca <
0 ro -0 E
u u 0Ln 0
0 1- L- L- ro 4-J
-j o 0 0 x D
0 >
V)


u u

E
(0 cr) 0 L4-J (D
0 CL 4-J -C
c 4-J
0 ro 0
4- Ln 4
4-J 4-J 4 (D (a (n
Q) c 0
-3 Q) 0 -C
ro m u (D u
C: m W LLJ LLI 4 4ra Lli cr- (13 -C 0
0) (U (1) Q) E 4
U) E
u 4-1 4 4 0 4-J 0 4-J
4-J 0 4-1 -W L. Lo L- ro
0- 0 ru Uj CO
V) -i -j u -0 u





85



suggested for Pinon Jays (Balda and Bateman, 1972; Bateman and Balda, 1973). Roadrunners, Pinon Jays and the herons studied here all have very dark skin. The down on the herons is initially too sparse to cover all the skin. The nature of the plumage of Little Blue Herons and Louisiana Herons is illustrated by McVaugh (1973). Hudson et al. (1974) and especially Weber (1975) provide photographic evidence of exposed skin in Cattle Egret nestlings of various ages.

To illustrate how important solar brooding might be, two examples of heat loss will be given. For consistency, both examples utilize Cattle Egrets; the cooling constants are taken from Figure 11. Equation (10) relates the time it takes to cool to the cooling constant. Assume Ta = 28.90C in both examples, and that preferred Tb = 38.90C (see Table 9). Equation (17) can be rearranged so that:

Zn(Tb Ta)t = Zn(Tb Ta)i at but,

Zn(Tb Ta)t = kn(38.90C 28.90C) = 2.303 so,

zn (Tb Ta)t = 2.303 a.t and


(Tb Ta)t = e(2.303 a-t) therefore


(Tb)t = (Ta)t + e(2.303 a't) (18)
In the first example, a new hatchling (0 days old) is exposed to T a
a

28.90C for 10 min while its parent stands up to turn any other eggs in the nest, chase an intruder, or attempt to feed an older sibling. The cooling constant for the hatchling is 0.0627. Then from equation (18) after only 10 min, Tb has dropped to 34.2C. In the second example, a 6-day-old chick which still has dark skin showing between feather tracts, is the youngest of three nestlings; its oldest sibling is 10 days. The






86



parent has left the nest for an hour. Even at T a = 28.90C, the older two siblings do not huddle. From Figure 11, the cooling constant of the youngest bird is 0.0129. After 60 min, T b will have dropped to 33.50C.

The absorption of solar radiation-in both the examples above could prevent the large drop of T b. This is probably a function of the dark skin. It is interesting that the small pink-skinned chick of White Ibises hatches out completely covered in dense black down. Bartholomew and Dawson (1954) indicated that down may protect young birds, including Great Blue Herons (Ardea 7wroilds) from solar radiation. Although that must certainly be true once internal heat production is relatively high and the surface/volume ratio is relatively low, very young chicks may

face problems of heat loss rather than heat gain.

Linsdale (1936: 117) reported a correlation of nestl]ing plumage

(especially down) with insolation. Looking at 15 Great Basin icterids and fringillids, he noted that 'birds which nest in exposed situations and which live in hot regions have pale or pallid nestling plumages and nest linings which reflect'' solar radiation.

The reason why dark Little Blue Herons have young with white

plumage whereas dark Louisiana Herons have young with dark plumage has to do with nesting patterns and the transition from a problem of heat loss to one of heat gain. If adults leave their young unattended once the oldest nestling is about 12.6 days old, younger siblings will not yet be homeothermic. Even the older siblings will not be totally independent of their thermal environment. But the younger siblings will not be capable of leaving the nest very readily. If left alone under high solar intensity at high T the chicks might not survive. Their white






87,



plumage (see Figure I for Little Blue Heron fledgling) helps protect them by reflecting much of the solar radiation. The Louisiana Heron generally nests in the shade. Presumably nesting in the sun would require more parental attention to keep their older dark nestlings from overheating. One Louisiana Heron nest was found in the sun in 1974. The adults continued attentive behavior (mostly shading one nestling) for 23 days. Thjs is longer than observed for any other nest of any of the four species. Such intense parental care must necessarily reduce the amount of food gathering possible and so lower the number of young fledged from the nest.














CHAPTER 6

THE SIGNIFICANCE OF COLOR IN HERONS


The color of birds that nest colonially or feed in open areas has been discussed at least since Darwin (1896). Darwin (1896: 493) noted that sea birds have a white plumage more often than terrestrial birds. He suggested that these birds would be conspicuous in a white or black plumage; conspicuous coloration would aid the birds in locating one another for mating. Yapp (fide Siegfried, 1971a) has pointed out that the communal roosts of birds,are conspicuous. Craik (1944) contended that sea birds were white, or had white undersides, to reduce their contrast to the sky while in flight, enhancing their ability to surprise prey items in the ocean. However, Armstrong (1944) noted the large number of dark birds that dive for food, and Cowan (1971) refuted Craik experimentally.

The debate on color in these birds is epitomized by the controversy over color in herons, especially polymorphism in herons. Several herons are dimorphic: the Reef Herons (Egretta sacra and E. gularis), the Little Egret (E. garzetta), the Reddish Egret (Egretta [= Dichromanassa] rufescens) have white and dark morphs; even the Least Bittern has a rare melanistic phase (Meyerriecks, 1960). Two of the best known dimorphic herons occur in North America. The Great Blue Heron (Ardea herodias) has a white morph which is often given subspecific status (A. h. occidentalis) and apparently represents a


88










Caribbean population. Ardea herodizs appears to be the only dimorphic heron for which a genetic mechanism has been suggested (Mayr, 1956). The Little Blue Heron's dimorphism is unique in the Ardeidae because the white morph is the juven ilIe of the species. Darwin (1896: 494) incorrectly suggested this sort of dimorphism in a reef heron.

Meyerriecks. (1960) reviewed the arguments of several authors who believe color in herons is related to feeding habits. Murton (1971) is the most recent advocate of this view. He said that white or dark color are cryptic colors, white color being more common in herons that feed in open waters during the day, and dark color facilitating hunting in more closed areas. He also noted that dark herons tend to be more active hunters. Murton suggested that white juvenal plumage in Little Blue Herons is useful for feeding on insects in grassland areas as Cattle Egrets do. Meyerriecks (1960) and Recher (1972) discounted color

as an aid to feeding by pointing out the lack of behavioral differences between light and dark morphs of a species when feeding. This extends to the juvenile and adult Little Blue Herons which show no differences in manner of prey capture except that the juvenile is less efficient (Recher, 1969).

Murton (1971) postulated that polymorphism has evolved in herons occupying coastal habitats in the absence of closely related species ''which might otherwise partition the resources.'' This may be possible, but not for the reasons Murton suggested. He bel ieved that different colored morphs would exploit different food resources in the habitat; but the lack of differences in the feeding ecology and behavior of different morphs of a species belie this. However, apostatic selection,




90





(as discussed by Johnson and Brush, 1972) which promotes polymorphism in predators by reducing the ability of prey to generalize a single search image of a predator, could be occurring. This hypothesis would probably be easily tested experimentally.

Meyerriecks (1960) considered white color in herons to be conspicuous in order to facilitate social interactions. He noted that the most social herons are white. Recher (1972) stated that the white juvenal plumage of Little Blue Herons may be related to their post-breeding dispersal and the tendency of juveniles to feed in flocks at this time.

Recher (1972) noted a correlation between color polymorphism, coastal (actually, open) environments,' and tropical latitude. It is true that most polymorphism in herons is confined to low latitudes. Recher stated that in hot exposed environments, 1white-coloured herons ... may be better able to regulate their body temperatures than their dark siblings"' while feeding. Very few workers have looked to see whether white herons even normally hunt in direct sunlight during the heat of the day. Siegfried 01971b) found that feeding activity in Cattle Egrets is reduced during that part of the day. However, I have shown that white plumage certainly confers a degree of protection from insolation among nesting Cattle Egrets and Snowy Egrets and nestling Little Blue Herons. Meyerriecks (1960) rejected the notion that white color is useful in temperature regulation, because he has seen Little Blue Herons and Green Herons remain in the sun for hours. The basal metabolism of Green Herons is not known, but M6 for Little Blue Herons




I





is sufficiently low (see Chapter 4) that they are able to stay in the sun in spite of their color.

The various theories on color polymorphism in herons fall into four main classes: 1) Herons are conspicuously white in order to facilitate social interactions; 2) light and dark phases are cryptic to facilitate feeding; 3) light and dark phases are the result of apostatic selection which facilitates hunting by deemphasizing a search image that prey can use; 4) white color confers thermal stability in a hot environment. Of these, 2) is least likely, although it may operate for a few species. Experimentation is needed to test 3), but the lack of polymorphism at high latitudes raises a question about apostatic selection. Neither 1) nor 4) can be easily dismissed. Unfortunately 1) cannot be tested; 4) can be tested because it is a function of characteristics (e.g., reflectance and rate of metabolism) which can be measured. Although juvenal plumage in Little Blue Herons may enhance social interactions, it seems to be primarily a thermal adaptation (see Chapter 5). Whether white coloration in Snowy Egrets and Cattle Egrets is primarily for social facilitation, thermal stability, some other factor or some combination of these cannot be known with certainty. However, white plumage does contribute to the ability of these birds to nest in a hot sunny environment. The selective force(s) responsible for dark coloration in Louisiana Herons and adult Little Blue Herons is even more obscure. Nevertheless, as a consequence of their dark color, these birds can only live in a hot environment by having special' behavioral or physiological adaptations.















CHAPTER 7

CONCLUSIONS


Louisiana Herons, Snowy Egrets,-Cattle Egrets, and Little Blue

Herons nest together in a hot environment with high insolation. Cattle Egrets and Snowy Egrets are able to nest in the sun because 1) their white plumage reflects most of the incident solar radiation in the visible spectrum and about half of the near infrared radiation below 900 pim; and 2) they have a basal metabolism (M b) only 35 percent of that expected from their weight. The two dark herons reflect very little of the solar radiation to which they are exposed. In addition, Louisiana Herons also have a slightly higher M b than expected (107 percent); their nesting strategy is to build nests in the shade. The Little Blue Heron, *however, does not preferentially select nest sites in the shade. Instead, it shows a host of adaptations which allow it to nest successfully in sites exposed to the sun. The M b of Little Blue Herons is only 66 percent of that expected from their weight, a relationship shown for other dark birds in a hot environment as well. They are less attentive in incubating than white Cattle Egrets when their nests are in the sun at similar ambient temperature (T a), although attentiveness is about the same for the two species in the shade. Unlike Louisiana Herons, Little Blue Heron young have a white plumage which probably is of value in preventing overheating in the sun in the absence of a parent and before complete homeothermy has developed. Egg 92










temperature for all species is about 35'C. Paradoxically, all four herons hatch with heavily pigmented skin which continues to show at the apteria for over a week. This pigmentation is considered an adaptation for solar brooding when the nestlings are SO Small that heat loss is a greater problem than heat gain.

All four species show the same general form of behavioral

thermoregulation at the nest. Parents brood small nestlings for most of the first week. At high Ila) they may shade their young which are all capable of evaporative heat loss by gular flutter from their first day. As the nestlings age, their parents spend progressively less time on the nest, and temperature regulation increasingly depends upon their own behavior. At cool T small nestlings huddle, whereas at high Ta they spread apart and lie prone in their nest. By the time the parents begin spending long periods of time away from the nest, the chicks are old enough to leave the nest and change their thermal environment. The effectiveness of the complex of parental and nestling behaviors is Seen in that body temperature= 38.9'C in nestlings of all species regardless of Ta, age, or brood s ize.

Cooling curves show that short term homeothermy is gradually achieved by 12-13 days in all four species. Cattle Egret nestlings appear to develop homeothermy fastest, Louisiana Herons slowest. Growth

in all four species tentatively can be considered to conform to the logistic equation, although the slow Cattle Egret growth at later nestling ages makes this species nearly fit the Gompertz equation.




Full Text
56
(Rudegeair, 1975), and for single-sex incubators as diverse as Mallards
(Caldwell and Cornwell, 1975) and small passerines (White and Kinney,
197*0- This relationship is probably directly related to incubation.
As T approaches egg temperature, the gradient between them is reduced,
and longer periods of inattentiveness can be tolerated without risk of
a critical drop in egg temperature. For Little Blue Herons on shady nests
and all Cattle Egrets, time spent off the eggs was used, in decreasing
order of occurrence, to preen, adjust nest material or eggs, engage in
aggressive interactions with neighbors or interlopers, and gather new
nest material. All these activities but the last, which was very rare,
occur at or very close to the nest site. The most common behavior of
Little Blue Herons on sunny nests when off the eggs was to adopt a
"shading" posture as described by Bartholomew (1966). Here the bird's
back is to the sun, wings are drooped, and guiar flutter is common.
There is no reason, however, to believe that these birds are trying to
prevent their eggs from overheating; otherwise Cattle Egrets at the same
T and on sunny nests would show this behavior. More likely, these
Little Blue Herons are trying to prevent themselves from overheating
by posturally increasing their convective heat loss. Nevertheless, by
effecting this posture while standing over the nest, solar overheating of
the eggs is prevented. On a few occasions though, a Little Blue Heron
would walk off a sunlit nest and stand for a few minutes in the shade.
Postural responses to thermal stress in incubating adults have been
reported before by Howell and Bartholomew (1962) for Sooty Terns (Sterna
fuscata) and by Bartholomew (1966) for Masked Boobies (Sula dactylatra).
The thermal stress resulting from insolation on dark plumage was also


CHAPTER 4
APU!.T METABOLISM
The energetics of temperature regulation in endotherms can be
expressed as
M = C(Tj T ) (1)
b a
where M is metabolism and represents the heat produced, C is thermal
conductance, and .T, and T are body temperature and ambient temperature,
0 cl
respectively. The term C(T, T ) is an estimate of heat loss at low to
r a
moderate T, when evaporative heat loss is low. As such, C should be
CL
considered "wet" thermal conductance (McNab, 1374) since it Incorporates
a small evaporative heat loss element. At higher
M C (T T ) + LE
b a
(2)
where L is the latent heat of vaporization, E is the amount of water
lost: via evaporation, and C is "true" thermal conductance. Because
evaporative heat loss was not measured in this study and C is given as
£5 minimum value at only low to moderate T equation (1) will be used
to discuss the energetics of thermoregulation. in an endotherm1s thermal
neutral zone, metabolism is basal (Af^) and equation (l) becomes
14 nfrp rr
i'-7, O i 1 i J. Q }
U D X.
where Tg is the lower limit of thermoneutrality.
3)
An endotherm has then three variables to effect thermoregulation:
ip Cy and (Ti Tn). Any two of these factors can be modified, thus
O'
etting the third. Usually endotherms modify M, and C (McNab, 1366a,
20


A
20-
10
O
o ,
I
, -Q
2-
3.0
2.5-
. .o
'o 2.0-
.0
Ak
h-.
-P
A\
'4
\
\ a
\A
\
\
P
a
q
LITTLE BLUE HERON
AGE IN DAYS
10
A 5
A- 3

1 1 1 r~
10
30
50
70
i r
90
no
130
150
170
TIME (min)
''-J
U1
190


97
Linsdale, J. M. 1936. Coloration of downy young birds and of nest
linings. Condor 38: 111-117-
Lundy, H. 1969. A review of the effects of temperature, humidity,
turning and gaseous environment in the incubation on the
hatchability of the hen's egg. In The Fertility and Hatchability
of the Hen's Egg (T. C. Carter and B. M. Freeman, eds.) pp. 143~
176. Oliver and Boyd, Edinburgh.
Lustick, S. 1969. Bird energetics: effects of artificial radiation.
Science 163: 387~390.
Lustick, S. 1971. Plumage color and energetics. Condor 73: 121-122.
Lustick, S., S. Talbot, and E. L. Fox. 1970. Absorption of radiant
energy in Redwinged Blackbirds (Agelaius phoeniceus). Condor
72: 471 -473 -
Maxwell, G. R. II, and H. W. Kale, II. 1974. Population estimate of
breeding birds on a spoil island in the Indian River, Indian
River County, Florida. The Florida Field Naturalist 2: 32-39-
Maxwell, G. R., II, and H. W. Kale, II. 1977. Breeding ecology of
five species of herons in south Florida. Auk 94 (in press).
Mayr, E. 1956. Is the Great White Heron a good species? Auk 73:
71-77.
McManus, J. J., and C. M. Singer. 1975- Social thermoregulation in
the mongolian gerbil, Meriones unguiculatus. Bull. New Jersey
Acad. Sci. 20: 20-25.
McNab, B. K. 1966a. The metabolism of fossorial rodents: a study of
convergence. Ecology 47: 712-733.
McNab, B. K. 1966b. An analysis of the body temperatures of birds.
Condor 68: 47-55-
McNab, B. K. 1969- The economics of temperature regulation in neo
tropical bats. Comp. Biochem. Physiol. 31: 227-268.
McNab, B. K. 1970. Body weight and the energetics of temperature
regulation. J. Exp. Biol. 53: 329 348.
McNab, B. K. 1974. The energetics of endotherms. Ohio J. Science
74: 370-380.
McVaugh, W., Jr. 1972. The development of four North American herons.
Living Bird 11: 155-173-


85
suggested for Pinon Jays (Baida and Bateman, 1972; Bateman and Baida,
1973)- Roadrunners, Pinon Jays and the herons studied here all have
very dark skin. The down on the herons is initially too sparse to cover
all the skin. The nature of the plumage of Little Blue Herons and
Louisiana Herons is illustrated by HcVaugh (1973)- Hudson et al.
(197^) and especially Weber (1975) provide photographic evidence of exposed
skin in Cattle Egret nestlings of various ages.
To illustrate how important solar brooding might be, two examples
of heat loss will be given. For consistency, both examples utilize
Cattle Egrets; the cooling constants are taken from Figure 11.
Equation (10) relates the time it takes to cool to the cooling con
stant. Assume ?a = 28.9C in both examples, and that preferred Tb =
38.9C (see Table 9)- Equation (17) can be rearranged so that:
£n(2 Ta)t = in{Tb ~ Ta)i at but
n{Tb T ) = Jn(38.9C 28.9C) = 2.303 so,
In- T = 2.303 a-t and
/m \ (2.303 a-1) f
[T-, T ), = e therefore
b a't
(Tb)t = (Ta)t + e(2-303 a't] (18)
In the first example, a new hatchling (0 days old) is exposed to T=
28.9C for 10 min while its parent stands up to turn any other eggs in
the nest, chase an intruder, or attempt to feed an older sibling. The
cooling constant for the hatchling is 0.0627* Then from equation (l8)
after only 10 min, Tb has dropped to 3^-2C. In the second example, a
6-day-old chick which still has dark skin showing between feather tracts,
is the youngest of three nestlings; its oldest sibling is 10 days. The


9
than 200 g could be measured by putting some handy object on the
second pan of the scale.
Egg temperatures were monitored in the nest by pis'; g 3
synthetic egg In the nest. The synthetic egg was made from a heron*3.
egg shell filled with Dow-Corning medical Silastic 382 Elastomar
(Caider, 1971)- A thermocouple wire was implanted in the egg which
was put in the nest with the natural eggs. The thermocouple lead
was kept on a makeshift spool and could be unwound about 100 feet to
connect to a portable thermocouple meter (Minimite by Thermo Electric).
The parent could be watched safely from a distance while egg tempera
ture was simultaneously monitored.
Because the chicks of all four species studied hatch asynchronously
(see Chapter 5)* size differences were indicative of relative ages.
Thus, it was unnecessary to mark individual chicks. Nests were monitored
almost daily after about the twentieth day of incubation to record
hatching dates. Nestlings on their day of hatching were considered
0 days old.
Laboratory Techo ?ques
Feather reflectances were measured on a Bausch and tomb
Spectronic 2.0 spectrophotometer using a color analyzer reflectance
attachment. A block of magnesium carbonate was used as a white
reflectance standard. Reflectance was measured in feathers of intact
birds., which had been preserved in a freezer, over a 3^0 my-900 my
range.
Cooling curves were used to determine the time when horneothernry
was first established In the young of these species. One nestling


99
Shanholtzer, G. F., W. J. Kuenzel, and J. J. Mahoney. 1970. Twenty-
one years of the McKinney's Pond Rookery. Oriole 35: 23-28.
Siegfried, W. R. 1966a. The status of the Cattle Egret in South Africa
with notes on the neighbouring territories. Ostrich 37: 157l69.
Siegfried, W. R. 1966b. Age at which Cattle Egrets first breed.
Ostrich 37: 198.
Siegfried, W. R. 1968. Temperature variation in the Cattle Egret.
Ostrich 39: 150-154.
Siegfried, W. R. 1969- Energy metabolism of the Cattle Egret.
Zoologa Africana A: 265-273.
Siegfried, W. R. 1971a. Communal roosting of the Cattle Egret.
Trans, roy. Soc. S. Afr. 39: 4l9~443.
Siegfried, W. R. 1971b. Feeding activity of the Cattle Egret.
Ardea 59: 38-46.
Siegfried, W. R. 1972. Food requirements and growth of Cattle Egrets
in South Africa. Living Bird 11: 193-206.
Simpson, G. G., A. Roe, and R. C. Lewontin. I960. Quantitative
Zoology, 2nd ed. Harcourt, Brace and World, New York. 440 pp.
Skutch, A. F. 1962. The constancy of incubation. Wilson Bull. 74:
115-152.
Sokal, R. R., and F. J. Rohlf. 1969. Biometry. W. H. Freeman and
Company, San Francisco. 776 pp.
Sprunt, A., Jr. 1954. A hybrid between the Little Blue Heron and the
Snowy Egret. Auk 71: 314.
Teal, J. M. 1965- Nesting success of egrets and herons in Georgia.
WiIson Bull 77: 257-263.
Weber, W. J. 1975. Notes on Cattle Egret breeding. Auk 92: 111-
117.
Weller, M. W. 1957- Growth, weights, and plumages of the Redhead,
Aythya americana. Wilson Bull. 69: 5"38.
Wetmore, A. 1921. A study of the body temperature of birds. Smithson.
Mis. Coll. 72: 1-52.
White, F. N., and J. L. Kinney. 1974. Avian incubation. Science
186: 107-115-


CHAPTER 5
THERMOREGULATION AT THE NEST
If it is true that the relatively low metabolism of some adult
herons is, at least in part, a consequence of nesting in areas exposed
to high solar radiation, then behavioral modifications might also be
expected. The preference among Louisiana Herons for shaded nest sites
is one such modification. A field study of the nesting habits of
Louisiana Herons, Little Blue Herons, Cattle Egrets, and Snowy Egrets
with respect to thermoregulation revealed several behavioral differences
among these birds .
All four of these species build relatively open stick nests which
are unlined. They all lay eggs which are essentially indistinguishable
in color (light blue) and size. Varying clutch size in these species has
been reported in different years and localities ranging from 3 to 7 eggs
(Bent, 1926; Howell, 1932; Meanley, 1955; Teal, 1965; Dusi, 1966; Blaker,
1969; Hopkins and Murtn, 1969; Dusi and Dusi, 1970; Shanholtzer et al.,
1970; Weber, 1975), but all showed a clutch size of about 3 in the Biven's
Arm study area (Table 7). The young of all four species hatch asynchro
nously after similar incubation periods of 23 +_ 2 days (Palmer, 1962;
Weber, 1975).
One conspicuous difference, however, is in nestling plumage
coloration. The dark Little Blue Heron has young with white plumage
as do the white Snowy Egret and Cattle Egret. The relatively dark
Louisiana Heron has young with a dark brown plumage. Paradoxically,
50


from a given nest would be removed and brought to the laboratory only
1.5 miles away. Usually the bird was returned to the nest the same
day, often within h or 5 hours of its removal. On a few occasions,
heavy rain or darkness prevented this, but the bird would then be
returned shortly after dawn the following morning, A thermocouple
whose junction was encased In latex was inserted cloacally 1.5*3 cm
depending on the nestling's size, and the wire was taped to the
pygostyle region. The thermocouple was connected to a Honeywel1
chart recorder via a thermocouple reference junction. The nestling
was put into a chamber which was placed in a water bath at 40C. When
the nestling began sustained "large amplitude guiar flutter (body
temperature usually was within one degree of 40C) it was put in
another chamber which had been kept in a 17C water bath. Ambient
temperature was monitored with a Y$! probe Type 402 which was suspended
in the chamber. Cooling was continued at least 60 min or until body
temperature closely approximafeci ambient temperature.
Metabolism was measured in adult herons using a Beckman G-2
paramagnetic oxygen analyzer connected *n open circuit (Depocas and
Hart, 1957) Air flow of about 1400 cm/'ri'n was pulled through the
chamber and measured by a Brooks precision rotameter calibrated with a
500 cc Brooks volume-meter after carbon dioxide and water were removed.
Carbon dioxide was removed from the air with color indicator grade soda
i ime (8-14 mesh), and water was removed by color indicator regenerated
silica gel (grade H, type IV, 6-16 mesh). Only about 200 cm/min was
put into the analyzer. Temperature and barometric pressure were
recorded,and oxygen consumption was calculated at SIP. Body tempera*


ACKNOWLEDGMENTS
I would like to thank my committee for all their help over the
years: John Kaufmann who helped me get started on this project and who
supported me when I needed it; Brian McNab who opened up the world of
physiological ecology for me and whose constant stimulation helped make
me a different kind of scientist thafi 1 would have been otherwise;
David Johnston whose wealth of ornithological information was indis
pensable and was given freely; and James Lloyd with whom I spent many
hours in my early years here refining my ideas about evolution and
adaptation.
This dissertation would never have been possible without the
help of Donna Gil lis who typed for me, befriended me, and led me through
the maze of red tape that thes.es are heir to.
Herbert Kale provided me with many of my experimental animals and
helped me gain access to Riomar Island. James and Elizabeth Wing
provided easy access to Biven's Arm and comfort from fatigue and the
elements. Jon Bartholic, in the Fruit Crops Department, helped me to
procure equipment which was essential for measuring solar radiation at
my nest sites. To all these people I am greatly indebted.
Thomas Emmel, Chairman of Zoology, has recently been my greatest
source of encouragement. He has strengthened my resolve at critical
times, in the preparation of this dissertation.
Too many people helped me collect data in the field for me to
enumerate here. But 1 am especially grateful to Joyce Newman,


Table 6.
Comparative energetics of herons.3
An ima1
Mb/C
(Mb/C)e
F
m 6
Lb
m n
LZ
(Tb T%)
V-
*
LOU 2
16.425
20.721
0.733
40.7
25.8
14.9
23.3
16,8
42.3
LOU 3
17.159
19.715
0.870
40.9

---
--
--
S L 1
15*556
20.586
0,756
40.6
26.5
14.1
25.0
15-6
42. 1
SE 2
15.829
¡9.928
0.792
4o. 7
26.0
14.7
25.0
15.7
41.8
CE 4
14.808
19.641
0.754
40.9
28.0
12.9
26.4
14.5
41.3
CE 5
17.447
20.487
0.852
40.4
25.2
15.2
23.3
17.1
42.6
LB )
12.746
20.096
0.634
40.1
27.8
12.3
27.8
12.3
40.5
LB 2
14.105
19./89
0.713
40.5
'll .2
13.3
27.2
13.3
41.3
aSymbo1
"Body t
s as in Tables 2 and 3
emperatures at all T
uni ess
unless,
otherwise noted.
above thermoneutral
ity.
r
"Estimated at minimum thermal conductance.
^Estimated 5-4] + T^ (modified from McNab, 1970).


89
Caribbean population. Ardea herodias appears to be the only di
morphic heron for which a genetic mechanism has been suggested (Mayr,
1956). The Little Blue Heron's dimorphism is unique in the Ardeidae
because the white morph is the juvenile of the species. Darwin (1896:
494) incorrectly suggested this sort of dimorphism in a reef heron.
Meyerriecks (i960) reviewed the arguments of several authors who
believe color in herons is related to feeding habits. Murtn (1971)
is the most recent advocate of this view. He said that white or
dark color are cryptic colors, white color being more common in herons
that feed in open waters during the day, and dark color facilitating
hunting in more closed areas. He also noted that dark herons tend to be
more active hunters. Murtn suggested that white juvenal plumage in Little
Blue Herons is useful for feeding on insects in grassland areas as
Cattle Egrets do. Meyerriecks (I960) and Recher (1972) discounted color
as an aid to feeding by pointing out the lack of behavioral differences
between light and dark morphs of a species when feeding. This extends
to the juvenile and adult Little Blue Herons which show no differences
in manner of prey capture except that the juvenile is less efficient
(Recher, 1969).
Murtn (1971) postulated that polymorphism has evolved in herons
occupying coastal habitats in the absence of closely, related species
"which might otherwise partition the resources." This may be possible,
but not for the reasons Murtn suggested. He believed that different
colored morphs would exploit different food resources in the habitat;
but the lack of differences in the feeding ecology and behavior of
different morphs of a species belie this. However, apostatic selection,


RATE OF METABOLISM (cc02/ghr)
r43


Table 5.
Comparative measurements of conductance
in four species of herons.
An 1mas
^ a
''mi n
n
avg
^avg^rni n
LOU 2
0.0573
0.0575
0.993
LOU 3
0.0602
0. C 616
1.023 .
3E 1
0.0450
0.0464
1.031
SE 2
0.0549
0.0566
1.031
CE 4
0.0574
0.0641
1.117
CE 5
0.0427
0.0431
O
O
4_D
LB 1
0.0443
0.0482
1.076
LB 2
0.0475
0.0553
1.164
aC = C/W in Table 2.
min


60
a marked effect in nest siblings' competition for food. Rarely was the
third (or fourth) nestling able to compete with its older siblings, and
if it survived, it showed a slower rate of growth than the first two
(Blaker, 1969; Siegfried, 1972). Ohmart (1973) demonstrated that asyn
chronous hatching in Roadrunners allowed regulation of the brood size
in relation to food supply.
After hatching, attentiveness should be defined as time spent at
or nearby the nest. The parents continuously attend the young after
hatching until the eldest is at least one week old. Four Louisiana Heron
nests, 14 Cattle Egret nests, and three Little Blue Heron nests were
watched at random hours every day after their eggs hatched for evidence
of parental absence. The first parental absence from the nests was
observed when the oldest nestling was 12.6 +. 0-7 days (range 8-19 days),
with no apparent differences among the three species. Most of the nests
had two nestlings, although only one nestling occurred in one nest of each
species. Because observations were not continuous, 12.6 days is probably
an overestimate of the eldest nestling's age at first parental absence.
Weber (1975) noted continuous attentiveness for about \b days in Cattle
Egrets whereas Blaker (1969) noted that no nest is left "unguarded"
before chicks are 10 days old. In three nests with twins, Blaker noted
continuous adu11 presence for an average of 13 days, and in three singleton
nests he noted continuous attentiveness for an average of 16.7 days. One
Louisiana Heron nest is not calculated into the mean nestling age. Its
only young was 23 days old at first parental absence; the significance of
this nest will be discussed later. The first parental absence does not
signify an end to the attentive period. On the contrary, the parents


Si-
demonstrated that thermal conductance fell as brood size increased.
Huddling probably acts only in reducing heat loss by decreasing the
effective surface/volume ratio.
Dunn (1975) in a review found that growth rate is the best pre
dictor of the age of endothermy (although many of her sources provide
age of homeothermy, not true endothermy). Growth in all four species
of herons is given in Table 11. These values were converted to growth
constants (K.) using the method provided by Ricklefs (1967)- The three
Egretta species seem to fit the logistic equation best. The Cattle
Egret shows a slowing of growth in the later ages measured, which is a
characteristic of the Gompertz equation (Ricklefs, 1967)- Nonetheless,
the Cattle Egret fits the logistic equation slightly better. This is
interesting because the Gray Heron (Ardea cinrea) and the Green Heron
(Butorides virescens) fit the Gompertz equation (Ricklefs, 1968). The
data provided in Table 11, especially for the Little Blue Heron and
Snowy Egret, cover too small an age range to claim one equation with
certainty. However, unless later growth is very different, all species
do seem to fit only the logistic model. The asymptotic weight in the
Ricklefs model could not be determined by strictly graphic methods,
due to the lack of growth data for older nestlings. Instead an asymptote
was selected to give the highest correlation coefficient (r) when age was
plotted against the logistic conversion factors. The asymptotes are
presented with the growth constants in Table 12. The Little Blue Heron
(430 g) and Louisiana Heron (A20 g) asymptotes probably do approximate
the upper limits of adult weight. However, 530 g is certainly an over
estimate for the Snowy Egret. Even so, if the Snowy Egret had had an


CHAPTER 3
NEST SITES AND SOLAR RADIATION
The nest sites of herons have been investigated before, but
usually with respect to height, vegetation type, or relative placement
when in a mixed species heronry {of. Bent, 1326 and Palmer, 1962).
Because solar radiation at the nest had never been measured before this
study, a confused picture of nest site selection in herons existed.
At Biven's Arm, where most of this study was done, the same general
nesting pattern was seen every breeding season from 1972 through 1375*
Louisiana Herons, which arrived first, tended to nest close to open
water. Little Blue Herons, in those years when they were present in
relatively abundant numbers, tended to aggregate in groups of three to
six or more nests, somewhat away from open water. Cattle Egrets, which
were the last to nest in large numbers, chose nest sites without any
obvious reference to open water. Snowy Egrets also appeared catholic in
"their selection of nest sites, but too few nested at Biven's Arm to make
definitive statement. All four herons nested in red maple or eider, the
two most, common trees, from 1 -53 m above the ground or water.
Unfortunately, the internal consistency at Biven's Arm from year
to year sheds no light on the observed nesting habits of these herons
elsewhere. For example. Bent (1926) found Little Blue Heron nests on
the outskirts of willow islands, onvy 2-4 feet above the ground. Or
the other hand, Mean. 1 ey (¡955) noted that Little Blue Herons nests
i 2


against the sun!s radiation: C is still too high. A low C, however,
may have an entirely different function. As a consequence of equation
(3),
'Vo = h-T-, (, That is, the zone of thermoneutrality is a function of M, and C. A
reduction in would lower T^) unless compensated by a lower
C. Thus a lower C may be the means by which an endotherm maintains
a relatively wide thermoneutr.al zone even while decreasing its heat
production. This may explain the low C value of Little Blue Herons.
Even so, its zone (13-4 +_ 0.7C) is significantly smaller (P < 0.05
using Students i) than any of the other species (16.2 + 0.7C) when
calculated as M*/C, An analogous situation may be found in Poor-wills.
Poor-wills nest in the sun and have a very low (49 percent)
(Bartholomew et at., 1962). They also have a C of 94 percent. This
C probably is not so important for its insulative properties as it
Is in guaranteeing a wider thermoneutral zone, for example, the
thermoneutral zone of a Poor-will is 6.5oC as calculated by M^/C.
1 f the Poor-will's C had been ICO percent, bU/C = 6.18C. Higher C,
D
though good for heat dissipation would narrow further this species'
ability to cope with its environment.
Empirically determined (T, T0) in Little Blue Herons (12.8 +_
0.5C) is not significantly different from that of the other herons
(14.4 + 0.4C). This Is due to the high Tp (28.0C) of CE 4. An estimate
of the lower limit of thermoneutral?ty at minimum thermal conductance
(?, ) is given in Table 6. Because the changes in C with decreasing


THERMOREGULATION IN FOUR SPECIES OF NESTING HERONS
By
HUGH IRL ELLIS
A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF
THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
1976


I certify that 1 have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
Fessor of Zoology
I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy. ,
1 (l L_
B. K. McNab, Co-Chairman
Professor of Zoology
1 certify that 1 have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
David W. Johnst/pn
Professor of Zoology
I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
-7.-
C
4
'dmes E. Lloyd
Professor of Entomology
Pi
V


feathers may be erected to
runners do not have dark plumage, but their
expose their dark skin when basking. Ohrnart and Laslewski showed that
although solar radiation was used to raise T^ in hypothermic Road-
runners, it was used to lower T by reducing conductance in normo-
thermic birds (much as in the birds studied by Hamilton and Heppner
and by Lustick). By comparison, Little Blue Herons were never observed
to bask in the sun (that is no special basking posture or orientation
was noted), and there is no indication of hypothermia in this species.
Table 4 shows metabolic rate as a function of the expected rate
from equation (5) for several dark nonpasserines which live in warm,
mostly tropical climates; included is the Roadrunner which utilizes
its dark skin for basking. if the data of Ohrnart and Lasiewski
(1371) are more accurate than those of Calder and Schrni dt-Niel sen (1967)
a low M-u is invariably correlated in dark birds with hot environments.
McNab (199, 1974) has noted a correlation between unpredictable food
sources and low M-- Both Turkey Vultures (Heath, 1962) and Roadrunners
(Ohrnart and Lasiewski, ¡973; Ohrnart, 1973) have unpredictable food supplies
as probably do Black Vultures; Little Blue Herons (Jenni, 1963; and others)
and probably Msgnificient Fngatebirds do wot. However, Roadrunners
(Ohrnart, 1973), Magni f icient Fngatebirds (pars, obs.) and Little Blue
Herons all may nest in the cun. Vultures do
may feed in full sun (pars. obs.). So while
a low in dark birds in hot environments (T
may be noted that the tempersta. Brow.i-headed
sun)saht to lower metabolic outout at T < T
" a
not nest in the sun but
food habits may contribute to
> T'q)'. in this regard it
Cowbird, wh!ch can uti Iize
(Lustick, 1369), does not hav


Table
12. Comparative
growth in herons
using the
logistic
a
equation.
Spec ies
Location
. b
Asymptote
KC
Hd
Source
Louisiana Heron
Florida
420
0.301
-0.173
P resen t
study.
Little Blue Heron
Florida
430
0.238
-0.186
Present
study.
Snowy Egret
Florida
530
0.312
-0.226
Present
study.
Cattle Egret
Florida
290
0.248
-0.261
P resen t
study.
Cattle Egret
Texas
270
0.232

Hudson
et dl. 1974
Cattle Egret
South Africa
300
0.326
Hudson et at,. 1974
(based on data from
Siegfried, 1972).
aFrom Ricklefs, 1967-
Estimated asymptotic weight in g.
c
Growth constant.
dRate of homeothermic development in days


Figure i .
Feather reflectances in herons.


Little Blue Herons spend much less time incubating when exposed to the
sun than do white Cattle Egrets. This behavioral response is thought to
be a consequence of their dark plumage. Nestling T% is nearly 39C for
o
all soeeies regardless of age, brood size, or 7 as a result of 1) both
CL
parental and nestling behavior, which is described, and 2) the ability
of nestlings to dissipate heat by evaporative means. Cooling curves
are used to determine the development of homeothermy which in these
herons is a function of age, not weight. Short-term homeothermy is
achieved by S3 days, although there is some indication that Louisiana
Herons achieve homeothermy later than the other three species. Growth
rates are compared and all are tentatively considered to fit the
logistic equation, although the Cattle Egret nearly conforms to the
Gompertz equation. Possible thermal reasons are considered for the
dark skin color in all four species and the white plumage of young
Little Blue
Herons.


REFERENCES
American Ornithologists' Union. 1957- Check-list of North American Birds.
5th ed. Amer. Ornithol. Union, Baltimore. 691 pp.
Armstrong, E. A 1944. White plumage of sea birds. Nature 153:
527-528.
Aschoff, J. and H. Pohl. 1970. Der Ruheumsatz von Vogeln als Function
der Tageszeit und der Korpergrosse. J. Ornithologie 111: 38-47-
Balda, R. P. and G. C. Bateman. 1972. The breeding biology of the
Pinon Jay. Living Bird 11: 5~41.
Bartholomew, G. A. 1966. The role of behavior in the temperature regu
lation of the Masked Booby. Condor 68: 523535
Bartholomew, G. A., and W. R. Dawson. 1954. Temperature regulation in
young pelicans, herons, and gulfs. Ecology 35: 466-472.
Bartholomew, G. A., J. W. Hudson, and T. R. Howell. 1962. Body
temperature, oxygen consumption, evaporative water loss and heat
rate in the Poor-will. Condor 64: 117-125.
Bateman, G. C., and R. P. Baida. 1973- Growth, development, and food
habits of young Pin Jays. Auk 90: 39"6l.
Benedict, F. G., and E. L. Fox. 1927- The gaseous metabolism of large
wild birds under aviary conditions. Proc. Amer. Philos. Soc.
66: 511-534.
Bent, A. C. 1926. Life histories of North American marsh birds. U.S.
Nat. Mus. Bull. 135: 1-490.
Blaker, D. 1969- Behaviour of the Cattle Egret, AvdeoXa ibis. Ostrich
40: 75-129-
Brody, S. 1949. Bioenergetics and Growth. Reinhold, New York.
Calder, W. A. 1971. Temperature relationships and nesting of the Calliope
Hummingbird. Condor 73: 314-321.
Calder, W. A., and K. Schmidt-Nie1 sen. 1967- Temperature regulation and
evaporation in the pigeon and the roadrunner. Am. J. Physiol.
213: 883-889.
Calder, P. J. and G. W. Cornwell. 1975- Incubation behavior and
temperatures of the Mallard Duck. Auk 92: 706-731 -
94


62
et al. (197^) show that shivering is not effective in Cattle Egrets
until they are about 5 days old.
As the nestlings grew older, parental attentiveness less often took
the form of brooding and more often consisted of standing one or two
meters away from the nest. During this period the behavior of the
nestlings played a major role in determining their thermal state.
Young nestlings were capable of moving about in the nest. Because many
nests had at least a partial exposure to the sun during parts of the
day, the chicks could often move in and out of the sun by moving only a
short distance. This situation is also seen, but somewhat more elaborately,
in Roadrunners whose nests are apparently built to provide both sun and
shade (Ohmart, 1973). During cool or windy periods when the adult was
off the nest, the young herons would huddle together. This behavior was
first noted in Cattle Egrets by Blaker (1969) and was observed by me
more often in this species than in the other herons.
At high T when the adult was off the nest, the nestlings of all
four species often responded behavi ora 11y. The nestlings moved apart
and spread out in a prone position, sometimes allowing their heads to
hang' outside the nest itself. This posturing probably increased their
surface/volume ratio and thus increased convective heat loss. As the
nestlings grew larger this prone posture gave way to the posture described
above for adults as "shading:" back to sun, wings drooped. The most
commonly observed response in nestlings to high Twas guiar fluttering.
All four species show the capacity to guiar flutter from hatching. This
has been reported before for Cattle Egrets by Blaker (1969) and Hudson
et al. (197^0 Hudson et at. showed that young nestlings could dissipate




83
asymptote of 420 g, K would increase only 3 percent from 0.312 to
0.322. The asymptote for the Cattle Egret is certainly an underestimate
of adult weight in the field. Hudson et at. (1974) equated their
asymptote with fledging weight, but I see no a priori reason for doing
this. The asymptotic weights for Cattle Egrets measured in Florida in
this study agree with those provided by Hudson et at. for Texas and
South Africa (based on data from Siegfried, 1972).
Table 12 also shows the rates of development of homeothermy (H)
which are the slopes from Figure 12. Cattle Egrets in Florida (and
Texas) grow more slowly than the other species, and develop homeothermy
more rapidly. But this inverse relationship does not seem to hold among
the other herons. Over a wider range of species, Dunn (1975) found that
a positive relationship existed between growth rate and the development
of homeothermy.
Color in Nestling Herons
In very small, essentially ectothermic chicks, heat loss is a major
problem. As the bird grows, its surface/volume ratio decreases, its
metabolic rate increases, and its insulative plumage increases. At a
certain point, heat loss is no longer the main problem, but rather heat
gain becomes of primary concern. At this stage, metabolic heat pro
duction may be too high, surface/volume ratio too small, and insulation
too great to withstand long periods of solar radiation. Usually all of
these problems are controlled by parental or nestling behavior. But
occasionally, behavior will not suffice.
One way of coping with heat loss for short periods is solar
brooding. This has been reported for Roadrunners (Ohmart, 1973) and


100
Yarbrough, C. G. 1970. The development of endothermy in nestling
gray-crowned rosy finches, Leucosticte tephroootis griseonuoha.
Comp. Biochem. Physiol. 3^: 917_925-
Yarbrough, C. G. 1971. The influence of distribution and ecology on
the thermoregulation of small birds. Comp. Biochem. Physiol.
33b: 235-266.


Table 2
Hetabolie re 1 at ienships in
four species of herons.
Art t mo !
Mb/W
C/W
u ...
weight in o
'' basa 1 met abo i s n ¡ in cc 0 7/ghr
"Ytlnlnum thermal conductance in ccO^/g-hrC + (Sckal and Rohlf, 1969)
body temperature in C, where 7 < T
a x
lower limit of thermoneutra!S ty in C
o #
" n o t: e n o u g h a 31 a t o d e t e r m i n e T 9
rfi
o
LOU 2
3 £' 3 1
,/ .J *
4*
2.59
0,951
+
0.012
0.0579
~r
O
O
O
to
40.*
4
0.1
25.8
¡ OU 3
2/6 50
4**
5-75
1.033
+
0.014
0.0615
...
0.0025
39.7
+
0. i
1
5E 1
333.73
0. b 1
0.700
+
0.020
0.0450
-h
0.C018
*0.2
+
0. 1
26.5
2£ ?.
2.,*, 23
+
1.51
0.369
0.012
0.05*9
0.0019
*0.4
4
0. 1
26.0
f /
Vj t
271.67
f
3. OS
0.850
+
0.015
0.057*
4-
0,0020
*0,6
f
0.1
2 8.0
c :£ 5
326.79
4
1.04
0.7*5
4.
0.014
0.0*27
+
0.0012
39-9
0, 1
25.2
3 (
239-63
+
1.83
0.571

0.010
0.0**8
+
0.0025
*0.0
*r
0.1
O -7 A
/ 0
LB 2
281.06
~'r
1.57
0.670

0.02.5
0.0*75
+
0.0028
*0.*
2-
0.1
71.2
LOU Lujis!
lana Here
:n.
SH = Snov
;y £ q j- 8
t,
CE = Cattle
Egret,
LG
¡ Little
Blue He
ron


t
summer. !n 1974 and 1975, Double-crested Cormorants (Phalaorocorax
auritus) and a very few Great Egrets (Casmeroidus albus) roosted
at the southwest heronry at night. Boat-tailed Crackles (Cassidix
major) and Red-winged Blackbirds (Agelaius phoeniceus) nested on
the fringes of the heronry and less often within the heronry.
The Biven's Arm heronries were somewhat insulated from mammalian
predation by water. However, a raccoon (Procyon lotor) was sighted in
a tree in the northeast heronry on April 15, 1972, emptying two Little
Blue Heron nests. Another raccoon was seen on the southwest shore next
to island 2 on May 9, 1973- Crows (Corvus spp.) were often seen at the
height of the breeding season. Owls were apparently active predators in
the heronry: owl pellets were found with piles of white feathers on the
southwest shore. Alligators {Alligator mis sissippiensis) presumably ate
young herons that fell out of their nests and perhaps adults drinking
water below their nests. No snakes were ever seen in the heronries.
Possibly Black-crowned Night Herons are occasional predators (Rudegeair,
Rlomar island is a spoil island with a large heronry described
by Maxwell and Kale (1977)- it has large Australian Pine trees
{Casuar ina. equisetifolia), but the rookery occupies only mangrove
trees, primarily black mangrove (Avicennia ntida) and white man
grove [Lagunaalaria, raoemosa) Most of the birds nesting with the
four species studied here are also arde ids, the major exception being
Brown Pelicans (Pelicanas oaaidentalic) which make up 11 percent of
the breeding birds (Maxwell and Kale, 197-4). Precation on Riomar
Island was by Fish Crows (Corvus oscifrage) and potentially rats
(Battue ratiue} according to Maxwell and Kale (1977).


Table 8. Heron egg temperatures.
a, b
LB-2
LOU 9
SE 3
CE 0
CE 108
x + s.e.
Pre-stabi1¡zat i on
31-9+0.4
9
31-7+0.4 31-1+0.2 32.4+0.4
10
Post-stab i 1 izat¡on
x + s.e. 34.9 + 0.2 34.9 + 0.4 35.2 + 0.3 34,7 + 0.2 34.4 + 0.3
17
11
15
16
23
Number of eggs in clutch
before synthetic egg added
Symbols as in Table 2; numbers refer to nest.
b
Measurements in C.


CHAPTER 7
CONCLUSIONS
Louisiana Herons, Snowy Eg rets, ,Ca111 e Egrets, and Little Blue
Herons nest together in a hot environment with high insolation. Cattle
Egrets and Snowy Egrets are able to nest in the sun because l) their
white plumage reflects most of the incident solar radiation in the
visible spectrum and about half of the near infrared radiation below
900 pm; and 2) they have a basal metabolism (A/,) only 85 percent of
that expected from their weight. The two dark herons reflect very
little of the solar radiation to which they are exposed. In addition,
Louisiana Herons also have a slightly higher than expected (107
percent); their nesting strategy is to build nests in the shade. The
Little Blue Heron, however, does not preferentially select nest sites
in the shade. Instead, it shows a host of adaptations which allow it
to nest successfully in sites exposed to the sun. The M^ of Little
Blue Herons is only 66 percent of that expected from their weight, a
relationship shown for other dark birds in a hot environment as well.
They are less attentive in incubating than white Cattle Egrets when
their nests are in the sun at similar ambient temperature (!T ), al
though attentiveness is about the same for the two species in the shade.
Unlike Louisiana Herons, Little Blue Heron young have a white plumage
which probably is of value in preventing overheating in the sun in the
absence of a parent and before complete homeothermy has developed. Egg
92


3
lentiginosus); Siegfried (1969) measured the existence metabolism for
Cattle Egrets at one temperature which was below their thermoneutral zone;
Bartholomew and Dawson (195^0 did some preliminary work on thermoregulation
in nestling Great Blue Herons; and Hudson et al, (197M described the
development of endothermy in nestling Cattle Egrets. No comprehensive
study has been done integrating the thermal requirements of adult and
nestling herons with their actual conditions in the field. This study
attempts to explain the thermoregulation of herons in the context of
nesting in a hot environment.


¡5
the proportion of highly exposed nests is compared in Table 1, The
2
value of 1.200 Langleys/min (= ca!/cm -min) ss chosen for strictly
heuristic reasons as a cut-off in the sun-shade continuum. Nests
receiving 1.200 Langleys or more are considered to be under intense
insolation. Only 6 percent of the Louisiana Herons fali into this
category, whereas at least: 20 percent of all the other species do.
An a priori comparison of solar radiation at nests between Louisiana
Herons and all the other herons yielded an Fvalue (Sokal and Rohif,
I9&9) which was highly significant {P 0.01).
This difference in nest sites is important, in Florida because all
these herons nest in the spring when average insolation is greatest, and
birds at exposed nests are subject to great thermal stress. By the time
the summer rains begin, much of the nesting season is over. The eventual
fate of most Louisiana Heron nests which were found in unshaded areas
is unknown. However, one Louisiana Heron nest completely exposed to the
sun was watched in 1974 and is described in Chapter 5. Bent (1926)
implies, but does not actually state, that large numbers of Louisiana
Herons may nest in the sun. The high energetic price that would be paid
for this behavior by the adults is discussed in Chapter 4; the possible
affect on nesting success is referred to in Chapter 5-
The orientation of nests in reference to an environmental parameter
is well known. Pi non Jays (Gymnovhinus oyanocephaZus) nest on the south
side of trees in order to utilize the heat from solar radiation (Baida
and Bateman, 1972). Desert Larks (.Arnmomanes deserti deserti) in the
Negev desert build nests facing north and sheltered from the mid-day
sun by rocks or vegetation (Orr, 970/. Calliope Hummingbirds (Stellula,
calliope) prevent nighttime heat loss by radiation to the heat sink of the



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PAGE 111

V9 PM \!n f n7 A


RATE OF METABOLISM (cc02/g-hr)
r43
(Do) qi


34
2) This metabolic economy results from a decrease in the thermal gradient
from the skin surface to the feather surface, thus lowering conductance
(first suggested by Cowles, 136'/; demonstrated by Lustfck, 1969; Hsppner,
1970; and Lustick et at. 1970), 3) This reduction in conductance may
result in a lowered and probably a lowered upper limit of thermo-
neutrality (Lustick, 1969), 4) All of this depends on an increased
absorption of energy in the visible (Lustick, 1969; and Heppner, 1970)
and perhaps near infrared spectrum (Lustick, 1969) by dark as opposed
to white feathers. 5) it is suggested that dark birds stay out of the
direct sunlight at T > T0 (Lustick et at., 1970) due to the earlier
mentioned reversed thermal gradient between skin and feathers to which
they would be subjected.
This last point is of particular interest because Little Blue
Herons often nest in the sun at ambient temperatures well above their
lower limit of thermoneutrality. It is not unusual for T to exceed
a
35C In the field, whereas T^ is 27.5C (see Figure 5). Furthermore,
incident solar radiation can easily exceed 0.9 cal/cm -min (Lustick,
?
2 .
1 -mi
or even 1.23 cal/cm min (Heppner, !?70). Solar radiation
measured at nests (including those, of Little Blue Herons) often exceeded
2 2
1.30 cal/cm min. On May 26, 1373, a maximum of 1.43 cal/cm min was
recorded at 1315 hours on Biverbs Arm.
Very few birds having dark feathers or skin and inhabiting hot
environments have been investigated. Two of those may have been hypothermic
and may have absorbed the sun's radiation to raise T^. Heath (1962) and
Ohmart and Lasiewski (1370 suggested this for Turkey Vultures (Cathavies
auva) and Road runners (Geoeooc-yx aat-ifornianus) respectively,
Road-


Abstract of Dissertation Presented to the Graduate Council
of the University of Florida in Partial Fulfillment of the Requirements
for the Degree of Doctor of Philosophy
THERMOREGULATION IN FOUR SPECIES OF NESTING HERONS
By
Hugh Iri Ellis
June, 197d
Chairman: John H. Kaufmann
Co-Chairman: Brian K. McNab
Major Department: Zoology
Temperature regulation in Louisiana Herons, Little Blue Herons,
Cattle Egrets and Snowy Egrets is compared to investigate any different
thermal requirements of these birds during nesting. Louisiana Herons
and adult: Little Blue Herons have dark plumage; Snowy Egrets and Cattle
Egrets are white or nearly white. Louisiana Herons nest consistently
in the shade. The other three species nest without regard to solar
input, hence may nest in the sun.
Metabolic rates of adults are compared. Louisiana Herons have a
high the two white herons have a somewhat lower M-, than expected
from their weight, and Little Blue Herons have a M, only 2/3 that
D
expected. The low M-, of Little Blue Herons is considered an adaptation
to nesting in the sun with dark plumage in a hot environment. St Is
shown that low M-, is characteristic of dark birds that have been stud Ie
in such err/i ronments.
to
Temperature regulation at the nest is investigated with regard
cubatinc: adults and nestlings, A stable egg temperature of 35C
appears to be Independent of parental attentiveness
at normal T .
17


Figure 2. Louisiana Heron metabolism.


8
hours Eastern Standard Time, when the sun was highest in the sky.
Measurements were made of nests at a variety of heights.
Several kinds of air temperatures were taken. Shade and sun
temperatures were usually taken with a Yellow Springs instrument (YSi)
telethermometer having 1C divisions and using a YSI Type 402 small
animal probe. Occasionally, these temperatures were taken with a
mercury thermometer having 1C divisions. In either case, the wind
was shielded from the device. Black body temperatures viere, measured
using a thermometer whose, bulb was painted with flat black paint and
dipped in black soot before drying. The black bulb thermometer, which
had 1C divisions, was wind-shielded by a tube painted flat black and
having a slot for exposure to the sun's rays. Black body readings
were always achieved by holding the thermometer normal to the sun's
rad ? ation.
Body temperatures of nestlings were measured using a YSI probe
Type 402. All body temperatures were, cloaca! and probe penetration
depth was 1-3 cm depending on the size of the chick. Occasionally
skin temperatures were taken using a YS banjo probe Type 409. If
a parent had to be disturbed from a nest in order to reach the
nestlings, temperature could usually be measured within 5 minutes
of adult departure.
Nestlings were usually weighed whenever possible to determine
growth character istics. The weight of small nestlings was taken with
a 100 g capacity Peso I
had an accuracy of + i
capacity double beam b
a spring scale having 1 g divisions
c. Larger nestlings were weighed on
alance with 0,1 g divisions. Weights
; the scale
a 200 g
greater


Table 7- 197^ clutch size at Biven's Arm
Louisiana Heron Little Blue Heron Snowy Egret Cattle Egret
x+s.e. 3-3+0.1
2.7 + 0.2
3.0 +0.4 3-0 + 0.1
N
17
7
4
26
vj-i


Cristina Palacio, Thomas Rudegeair, Laura Solomon, Ginny McCormack,
and Mary Paulic. One person, in particular, must be mentioned.
J. Perran Ross, my friend and colleague, spent months in the field and
in the laboratory with me. His hard work, stimulation, and criticism
have been indispensable.
Esta Belcher provided all of the illustrations in this paper. Any
errors in these figures are my own. Frank Nordlie kindly criticized
and read this paper.
The research for this work was supported in part by a grant from
the Frank M. Chapman Memorial Fund administered by the American Museum
of Natural History.
i i i


Figure 9. Snowy Egret cooling curves.


RATE OF METABOLISM (cc02/g-hr)
Tb ro


66
Blue nestlings measured for body temperature, only one was a singleton and
six were twins; no triplet nestlings were measured. No differences in TV
could be attributed to the number of young in the nest, Tor age in
any species, as reflected by the small variance around each mean 2^.
The very similar mean T^ among all the species is probably a consequence
of their similar behavioral responses to the thermal environment even
where the insolation varied greatly (see Chapter 3)- The values of T^
reported in Table 9 are about 1C below the *i0C reported for Cattle
Egret nestlings by Hudson et at. (197*0 Such a difference could
easily be caused by the 5 min period of parental absence due to my
disturbance.
The inability of young nestlings to maintain a high T^ in the
absence of a parent was evident in the following cases. On Hay 9, 197**,
a Cattle Egret nest (CE 4) in the shade (T = 30C) had three nestlings,
aged 6, 5, and 1 day old, all huddled together. T^ was 36.5C, 36.0C,
and 33-0C respectively. The parent had been scared away by my presence
an undetermined period of time earlier (>> 5 min). On May 19, 1975, an
adult Snowy Egret was flushed from its nest (SE 3)- After 15 min, the
2V of its singleton nestling was 35-3C. The nestling had hatched that
day and weighed only about 13 g. The lower T^ of the smaller chicks
in these two cases is related to their level of homeothermy as discussed
be 1ow.
Development of Homeothermy
Dawson and Hudson (1970) and Dunn (1975) review an extensive
literature on the development of homeothermy in birds. The only study of
the development of homeothermy in ardeids has been by Hudson et at.


which converge at 2^. C has been calculated based on the metabolic
response to the lowest T measured (Lasiewski et at., 1970). Calder
a
and Schmidt-Niel sen (1967) considered
C =
M LE
(T, T )'S
b a
05)
where S is surface area. This is a form of equation (2). Here is
true dry thermal conductance. Unfortunately, most of the literature
is based on "wet" thermal conductance (cf. Lasiewski et at,, 196?).
However, since Tis normally low, LE should be fairly small and C
should be only slightly less than C. Using the data Calder and
Schmidt-Niel sen (1967) present for the Roadrunner, calculated the
minimum slope C (extrapolated to T^) as 0.CO73 ccO^/q-hr-0Z (100
percent of expected). This differs from the average low TC* of
1.30 ca1/mfhrC (= 0.042 ccO^/ghr-C) by less than 12 percent.
A slightly more accurate calculation of C would involve using the
same data points which produce minimum conductance in a slightly different;
way. Recognizing that the extrapolation to involves an average 7.^,
each metabolic value can be regressed on its own A family of slopes,
whose average is an estimate of C, results. Table 5 shows the difference
between "minimum" conductance and "average" conductance; the former is a
slope itself, the latter is an average of values determined by several
slopes. !n seven out of eight cases in Table 5, the average value gives
a higher estimate of C. If these results can be generalized, all methods
of calculating C differently than the last averaging method, tend to
underestimate the parameter.


19
but. being dark, its reflectance curve is probably close to that of the
adu1t.
The differences in reflectance mean that a Little Blue Heron or
Louisiana Heron would reflect only about 6 percent of the solar radiation
striking it, whereas the white birds would reflect 80-85 percent of the
incident insolation. Feathers probably do not transmit much light.
But whether the non reflected radiation (1 r) is absorbed by the
feathers or transmitted directly to the skin, dark birds in direct
sunlight must be under a considerably increased thermal load. Louisiana
Herons have apparently adopted a strategy of nesting in the shade to
cope with this problem. Little Blue Herons have apparently developed
physiological and behavioral adaptations which allow it to nest in the
sun. Their strategies will be discussed in Chapters h and 5-


82
Table 11.
Growth in
0
nest 1ing herons.
b
Age
Lou isiana Heron
Snowy Egret
Cattle Egret
Little Blue Heron
0
19.4 + 0.4
21.5 + 2.0
19-7 + 0.7
22.0 + 0.6
07)
(2)
(8)
(3)
1
25.4 + 0.6
26.5
25-5 + 0.3
23.8 + 1.2
(16)
(1)
(11)
(2)
2
35-3 + 2.2
33.0
' 35.1 + 1.7
30.0
G)
(1)
(IT)
(1)
3
44.2 + 3- 1

42.1 + 3-8
39.0
(6)
(6)
(1)
4
60.0 + 5-2
71-5 +14.5
55.8 + 1.8
(9)
(2)
(8)
5
77-7 + 3-8
62.5
69.5 + 2.8
78.8 + 5.7
(9)
(1)
(3)
(2)
6

126.5
81.6 +4.3
(1)
(4)
7
--
109.1 + 15-4
8
9
(4)
173-0
135-5 + 7.4
(1)
(3)
10
211.5
151.5
(1)
(1)
11
182.0
(1)
12
206.0
(1)
13
178.2 + 7.2
(27
14
181.0
(1)
aMearr t* s.e.. measured in g.
Number in parentheses is sample size.


20
3.0-
2.5-
10-
2-
2.0-
1.5-
I .O
i I i I I :I 1 1 1 *l 1 1 1 1 1 I 1
10 30 50 70 90 110 130 150 170
TIME (min)
190


This dissertation was submitted to the Graduate Faculty of the Department
of Zoology in the College of Arts and Sciences and to the Graduate Council,
and was accepted as partial fulfillment of the requirements for the degree
of Doctor of Philosophy.
June, 1976
Dean, Graduate School


52
all four species hatch with only sparse down and heavily pigmented dark
blue (occasionally dark green in Cattle Egrets) skin. These differences
plus those already discussed in the adults are mainly responsible for
the differences in thermoregulatory behavior at the nest.
1ncubation
Attentiveness, or time spent on or at the nest is one of the most
important parameters of incubation. Unfortunately, this term connotes
different behaviors in different kinds of incubators. "Attentive period"
for single-sex intermittent incubators means the time spent at the nest,
whether on the eggs or not, as opposed to "inattentive period" spent in
foraging, maintenance activities and s'ocial interactions. This is the
usage of White and Kinney (197*0 even though they define attentive period
as the "time spent on_ the nest per unit time" (emphasis added). For
bisexual incubators where both parents alternate at the nest, the situation
is different: one parent is always present at the nest. The attentive
period for these birds is best considered as the time spent on the nest,
that is actually sitting on eggs. Then inattentive time becomes the
time not actually on the eggs; it is spent the same way as in single
sex intermittent incubators except for foraging. This distinction would
follow Skutch (1962) although not using his "session/recess" terms.
Attentiveness was examined for Little Blue Herons and Cattle Egrets
to assess the effect of plumage color on the incubating adults. The dark
Little Blue Herons and the nearly all white Cattle Egrets were selected
as representatives of the heron color spectrum. Nests in the shade were
compared to nests in the sun; the latter were inspected on sunny and


57
evidence by the fact that Little Blue Heron adults in the sun always
showed guiar flutter (a mechanism of evaporative heat loss) at lower T
than Cattle Egrets, Snowy Egrets, or White Ibises (pers. obs.). Further
more, Cattle Egrets on sunny nests showed the same postural "shading" as
Little Blue Herons, but only at more elevated levels of T .
a
Two Louisiana Heron nests were observed on two occasions. One
adult was on the eggs 96.8 percent of the time where the nest was shaded
from a cloudless sky at T = 28C. The other adult was on the eggs
100 percent of the time on a very cloudy day when T = 28C also. These
observations agreed with my general impression that Louisiana Herons,
which usually nest in the shade, are very attentive. However, no
Louisiana Herons that nested in the sun were observed (almost none
existed), and there are insufficient data to draw general conclusions
about attentive time for this species. Snowy Egrets were not observed
at all in this rega rd.
On three occasions, it rained during observations of Little Blue
Herons and Cattle Egrets. Attentive time increased to 100 percent each
time. This increase in attentiveness during rain is also reported by
Caldwell and Cornwell (1975) and Rudegeair (1975) -
Egg temperatures were monitored with a synthetic egg. In the
morning, egg temperatures were usually below 3^C and rarely fell as low
as 20C. Egg temperatures stabilized by around noon and remained around
35C regardless of attentiveness by the adult (see Table 8). The stabi
lized egg temperatures of these herons are relatively low compared to those
of most birds studied (Drent, 1973)- Lundy (1969) found in chickens that
egg temperatures below 35C reduced hatching success. The egg tempera-


96
Hamilton, W. J. Ill, and F. Heppner. 1967- Radiant solar energy and
the function of black homeotherm pigmentation: an hypothesis.
Science 155: 196-197.
Heath, J. E. 1962. Temperature fluctuation in the Turkey Vulture.
Condor 64: 23^-235 -
Heppner, F. 1970. The metabolic significance of differential
absorption of radiant energy by black and white birds. Condor
72: 50-59.
Hopkins, M. N., Jr., and P. G. Murtn. 1969. Rookery data from
south Georgia. Oriole 34: 1-11.
Howell, A. H. 1932. Florida Bird Life. Coward McCann, New York.
579 PP.
Howell, T. R., and G. A. Bartholomew. 1962. Temperature regulation in
the Sooty Tern, Sterna fuscata. Ibis 104: 98-105-
Hudson, J. W., W. R. Dawson, and R. W. Hill. 1974. Growth and develop
ment of temperature regulation i.n nestling cattle egrets. Comp.
Biochem. Physiol. 49A: 71 77^+1 -
Jenni, D. A. 1969- A study of the ecology of four species of herons
during the breeding season at Lake Alice, Alachua County, Florida.
Ecol. Monogr. 39: 245-270.
Johnson, N. K., and A. H. Brush. 1972. Analysis of polymorphism in the
Sooty-capped Bush Tanager. Syst. Zool 21: 245-262.
Jones, R. E. 1971. The incubation patch of birds. Biol. Rev. 46:
315-339-
King, J. R. 1964. Oxygen consumption and body temperature in relation
to ambient temperature in the White-crowned Sparrow. Comp. Biochem.
Physiol. 12: 13-24.
Lancaster, D. A. 1970. Breeding behavior of the Cattle Egret in
Colombia. Living Bird 9: 167- 194.
Lasiewski, R. C., and W. R. Dawson. 1967- A re-examination of the
relation between standard metabolic rate and body weight in birds.
Condor 69: 1323
Lasiewski, R. C., W. W. Weathers, and M. H. Bernstein. 1967- Physio
logical responses of the giant hummingbird, Patagona gigas.
Comp. Biochem. Physiol. 23: 797843-
Lasiewski, R. C., W. R. Dawson, and G. A. Bartholomew. 1970. Tempera
ture in the Little Papuan Frogmouth, Podarqus oaellatus. Condor
72: 332-338.


figure 6.
Metabolism vs. solar radiation in herons.


Figure 8. Louisiana Heron cooling curves.


7
Fle 1d Techniques
Observations at Biven's Arm were made from a rowboat, a tower,
occasionally from shore (only for the southwest Islands). A 15 foot
high tower with a 4"foot square observation platform 12 feet high was
attached to a raft 10 feet by 15 feet in size. The raft could be
moved by poling with a 16-foot pole which was equipped with a "duck
bill" at one end to allow bracing against the. soft mud bottom of
Biven's Arm. By shrouding the top of the tower with burlap a movable
blind-on-stilts was effected. Observations were made using 7 x 50
binoculars.
Nests on Biven's Arm were approached In a rowboat or a canoe
when it was necessary to tag them for identification, or to observe,
measure, or manipulate their contents. Many of the waterways in the
large northeast heronry of Biven's Arm were kept open by the activity
of alligators. Other paths clogged with cut-grass had to be cut open
and then frequently maintained. Nests on Plomar island were reached
by walking.
Solar measurements were made with a dome solarimeter (model no.
615 from Science Associates) which integrated over a wavelength
range of 0.3 u to 3.5 U (ultraviolet to far infrared) and which had
a sensitivity of 25.8 mV/Langley (1 Langley = 1 cal/cm)- The
solarimeter itself was mounted on a 5 foot aluminum pole bent at
75. At the opposite end of the pole a small bubble compass was
attached at 75 so that the bubble was centered when the solarimeter
was horizontal. Measurements were taken only between 1000 and 1400
or


64
became largely a function of nestling behavior. The four nestlings in
CE-106 were 6, 5, ^, and 3 days; the youngest nestling was a runt. At
1030 hours, the parent was off the nest, and the young birds huddled
together. At about 1055 the parent lay on the nestlings. Five minutes
later the sun reappeared, and the parent arose, briefly shaded its
chicks, then moved about 2 feet from the nest. By 1110, all of the young
had spread out assuming a prone position; two,which could be clearly
seen, guiar fluttered. After the sun disappeared behind clouds at 1120,
guiar flutter stopped. By 1130, the wind had picked up again and the
nestlings huddled together.
By the time heron nestlings are 9 or 10 days old, they are capable
of leaving the nest to climb on adjacent branches. These "branchers"
(Siegfried, 1966b) were first noted at 9 days old in Cattle Egrets by
Blaker (1969). The ability to move out of the nest has great significance
for the brancher in altering its thermal environment. By the time they
were 12-13 days old, the branchers were so adroit at leaving the nest,
they could rarely be caught.
The thermoregulatory effect of parental and then, increasingly,
nestling behavior can be seen by looking at nestling body temperatures.
Body temperatures of young birds were taken within 5 min of entering the
heronry. The results are shown in Table 9- Of the young Louisiana
Herons whose was measured, three came from singleton nests, 10 from
nests with twins, and 10 from nests with triplets. Of the Snowy Egret
nestlings, one was a singleton, the other two were twins. Two of the
Cattle Egret chicks in Table 9 were singletons, five were from nests
with twins, and nine were from nests with triplets. Among the Little


Figure 5-
Little Blue Heron metabolism.


$
sV
mj 2. 7 6
y>' 'T ^ 1


100-1
CO
o
O 80-
L


UJ 60-
1- -
h-
LU
O
cr 20-
LU
Q_
o L.B. NEST >50% SUNLIT
@ L.B. NEST<; 50% SUNLIT
C.E. NEST > 50% SUNLIT
a C.E. NEST < 50% SUNLIT
o MEASUREMENTS TAKEN
a SIMULTANEONSLY
0
31
25
27
29
-/
7 1 1 1 1
35 37 39
0
VJ1
\J~I


93
temperature for all species Is about 35C. Paradoxically, all four
herons hatch with heavily pigmented skin which continues to show at
the apteria for over a week. This pigmentation is considered an
adaptation for solar brooding when the nestlings are so small that
heat loss is a greater problem than heat gain.
All four species show the same general form of behavioral
thermoregulation at the nest. Parents brood small nestlings for most
of the first week. At high T they may shade their young which are
all capable of evaporative heat loss by guiar flutter from their
first day. As the nestlings age, their parents spend progressively less
time on the nest, and temperature regulation increasingly
depends upon their own behavior. At cool Tsmall nestlings
huddle, whereas at high T they spread apart and lie prone in their
nest. By the time the parents begin spending long periods of time
away from the nest, the chicks are old enough to leave the nest and
change their thermal environment. The effectiveness of the complex
of parental and nestling behaviors is seen in that body temperature =
38.9C in nestlings of all species regardless of T age, or brood
size.
Cooling curves show that short term homeothermy is gradually
achieved by 12-13 days in all four species. Cattle Egret nestlings
appear to develop homeothermy fastest, Louisiana Herons slowest. Growth
in all four species tentatively can be considered to conform to the
logistic equation, although the slow Cattle Egret growth at later
nestling ages makes this species nearly fit the Gompertz equation.


CHAPTER 6
THE SIGNIFICANCE OF COLOR IN HERONS
The color of birds that nest colonially or feed in open areas
has been discussed at least since Darwin (1896). Darwin (1896: b33)
noted that sea birds have a white plumage more often than terrestrial
birds. He suggested that these birds would be conspicuous in a white
or black plumage; conspicuous coloration would aid the birds in locat
ing one another for mating. Yapp (fide Siegfried, 1971a) has pointed
out that the communal roosts of birds,are conspicuous. Craik (1944)
contended that sea birds were white, or had white undersides, to reduce
their contrast to the sky while in flight, enhancing their ability
to surprise prey items in the ocean. However, Armstrong (1944)
noted the large number of dark birds that dive for food, and Cowan
(1971) refuted Craik experimentally.
The debate on color in these birds is epitomized by the controversy
over color in herons, especially polymorphism in herons. Several herons
are dimorphic: the Reef Herons (Egretta sacra and E. gularis) ,. the
Little Egret (E. garzetta), the Reddish Egret (Egretta
[= Dichromanassa] rufescens) have white and dark morphs; even
the Least Bittern has a rare melanistic phase (Meyerriecks, i960).
Two of the best known dimorphic herons occur in North America. The
Great Blue Heron (Ardea herodias) has a white morph which is often given
subspecific status {A. h. occidentalis) and apparently represents a
88


46
Body Temperature
it was stated earlier that once M~ and C were established (T-, Tn)
o b
was set. This is simply a consequence of equation (3). (2^ 2) <5 f
course itself not a single variable but Is the difference between two
variables, one of which may be selected for. T^ is usually higher for
birds than mammals of the same weight. This is a consequence of a
generally higher and lower C in birds (McNab, 1366b and 1S70) T^
and are related to each other and body weight. -This can be
expressed as
T, = 5 -4]F47-23 + T (16)
b i
where 5-41 '/P ^ ^ = 4.6 W ^ /0.8SW the expected ratio (M^/C) for
birds (Lasiewski and Dawson, 1967 and Lasiewsk! et at., 1967) and F is
the relative ratio between observed and expected Al^/C [equation (16) is
modified from McNab, 1970]. Table 6 shows that is close to that
predicted by equation (16). measured for Cattle Egrets in this study
(40.6C) agrees well with the T- for wile and cap+ive Cattle Egrets
(40.46C) reported by Siegfried (1968). Wetmore (1921) reported Snowy
Egret T-, at about 40C which corresponds closely to the 40,6C reported
.0
here. Table 6 also shows that the differences in (24 Ty) between
Little Blue Herons and the ether three species mentioned earlier is due
to differences in T^. When 2^ of Iwttle Blue Herons (40.3 + 0.2C) is
compared to an average T^ for Loui
iana Herons,
Snowy Egrets and Cattle
Egrets (40.5 - Q.2rC), there
when comparing Tn But T^ for
than the other species (27-5 +
is no significant difference nor is there
Little Blue Herons is significantly higher
0-3C vs. 4.7 + 0.5C, F < 0.05) using a


Figure 10. Cattle Egret cooling curves.


2
(Dickerman and Parkes, 1968, Parkas, pers, con:m.) Dicker-nan and
Parkes noted hybridization of Louisiana Herons with Little Blue Herons
and with Snowy Egrets They also cited a note by Sprunt (1954) of a
hybrid from a Little Slue Heron and a Snowy Egret cross. Curry-Lindahl
(19/1) suggested a reclassification of the Ardeidae on the basis
of behavior anc! ecology. He too suggested that Little Blue Herons,
Louisiana Herons and Snowy Egrets all belonged in Egr-etta,
All four species of herons have mainly tropical to subtropical
breeding distributions, although a larger subspecies of the Snowy
Egret {Egvetta ihulo bvewstebi) nests in several of the western
states of the United States (Bent, 1926; Palmer, 1962). The Cattle
Egret has recently invaded temperate areas in North America {Palmer, 1962;
Weber, 1975), Australia (Palmer, 1962), and southern Africa (Siegfried,
1966a). The Cattle Egret invasion of temperate zones has mostly
occurred since 1920 and is apparently a response to increased pasture
land (Siegfried, 1966a). The tropical/subtropical distributions alone of
these intermediate-sized nonpasserines makes them an attractive subject
for study; n¡ost nonpasserines studied are temperate species.
All four of these herons nest together in mixed species heronries
throughout Florida and the rest, of the Gulf coast. states. in Florida they nes
in Spring when solar insolation is most; intense. Their different, colors
under strong solar radiation must affect their nesting patterns in
some way, yet no one nas looked at nesting in herons from a thermal
viewpoint. In fact, relatively little is known about; their thermoregulation
at all. Benedict and Fox (1927) reported rates of metabolism for the
Great Blue Heron (Ardea kerod'ic) and the American Pattern (Botauvus


59
ture of the Herring Gull (Larus argentatus) which like arde ids has bi
sexual incubation, is about 38C (Drent, 1970).
These apparently low egg temperatures may be the result of using a
synthetic egg. The thermocouple from the egg left through the bottom of
the nest. Perhaps because it could not be turned or otherwise moved, the
synthetic egg was usually pushed to the bottom of the nest. At least
once the egg was pushed nearly completely through the bottom of the nest.
An egg pushed down in the nest would not only be farther from the
incubating adult and any incoming solar radiation, but also it would be
subject to updrafts and winds through the open weave st i ck nest. This
would be exaggerated in the cool morning when the birds are moving off the
eggs occasionally. Egg temperature can be adjusted by the tightness of
sit (White and Kinney, 197^)- Tightness of sit can be expected to increase
at night when the adult settles down to sleep. One nest, CE 108, was
monitored one night from 2300 hours to 0500 hours the following morning.
During this period, the adult never got off the eggs and egg temperature
stayed within a degree of 35C. This tends to confirm the hypothesis that
the morning egg temperatures were low as a result of the synthetic egg
being pushed to the bottom of the nest and in the absence of good tight
ness of sit.
There is no evidence of the presence or absence of incubation patches
in Ardeidae in any of the literature searched (e.gJones, 1971)-
Behavioral Temperature Regulation
Chicks of all four heron species usually hatch asynchronously.
Blaker (1969) showed that asynchronous hatching in Cattle Egrets had


CHAPTER 2
METHODS AND MATERIALS
Study Areas
Little Blue Herons, Louisiana Herons, Snowy Egrets, 2nd Cattle
Egrets were studied from sipring 1971 through 1975. Most of the field
work was done on Bivens Arm, a lake In Gainesville, Alachua County,
Florida. Some solar radiation measurements were made on Kiornar Island
in the Indian River, at Vero Beach, Indian River County, Florida.
Biven's Arm has been described by Carr (13^2), Rudegeair (1975),
end K'ord! i e (1976), It has about 65 hectares of open water (Nordlie,
1976) and is surrounded by pasture and swampy woodland. The current heronry
has existed on the northeast side since the early 1960!s and probably
represents the same heronry described by Jennl (19.69) at Lake Alice
1,5 miles northwest from 1958 to i960 (Nordlie, pers. comm.). Jenni
speculated that the. Lake Alice heronry had originally come from Biven's
Arm in 1348. in 1972 some of the herons from the northeast side of
biven's Arm built nests on a series of five small islands on the south
west side of the lake, The islands were numbered sequentially 1 through
5 starting at the north. By 1573 as many as kO percent of the herons on
Biven's Arm nested on the. southwest side of the lake. The southwest
islands were anchored, and only on their outskirts was the vegetation
partly floating or else slightly submerged. Some, if not ail, of the
Islands were at one time floating free in Biven's Arm, presumably
having been torn by winds ftom another part of the shoreline.
A
Aerial


is sufficiently low (see Chapter 4) that they are able to stay in the
sun in spite of their color.
The various theories on color polymorphism in herons fall into
four main classes: 1) Herons are conspicuously white in order to
facilitate social interactions; 2) light and dark phases are cryptic
to facilitate feeding; 3) light and dark phases are the result of
apostatic selection which facilitates hunting by deemphasizing a
search image that prey can use; 4) white color confers thermal stability
in a hot environment. Of these, 2) is least likely, although it may
operate for a few species. Experimentation is needed to test 3), but
the lack of polymorphism at high latitudes raises a question about
apostatic selection. Neither 1) nor 4) can be easily dismissed. Un
fortunately l) cannot be tested; 4) can be tested because it is a
function of characteristics [e.g., reflectance and rate of metabolism)
which can be measured. Although juvenal plumage in Little Blue Herons
may enhance social interactions, it seems to be primarily a thermal
adaptation (see Chapter 5). Whether white coloration in Snowy Egrets
and Cattle Egrets is primarily for social facilitation, thermal stability,
some other factor or some combination of these cannot be known with
certainty. However, white plumage does contribute to the ability of
these birds to nest in a hot sunny environment. The selective force(s)
responsible for dark coloration in Louisiana Herons and adult Little
Blue Herons is even more obscure. Nevertheless, as a consequence of
their dark color, these birds can only live in a hot environment by
having special' behavioral or physiological adaptations.


PERCENT REFLECTANCE
CO


are white or nearly white. Little Blue Herons had an average
spec ies
ja% of 66 percent of expected and have dark plumage. A few measure-
merits on a yearling Little Blue Heron with white plumage were equally
1 ow.
it is unlikely that differences in are phylogenetic; all the
species except the Cattle Egret are congeneric. Furthermore, as discussed
earlier, all have similar distributions. The differences ir M may be
directly attributable to thermal adaptations. Louisiana Herons, by
nesting in the shade encounter low solar heat loads. Snowy Egrets and
Cattle Egrets may nest in the sun but reflect a substantial amount of
the sun's visible radiation. What extra heat load they bear is at
least partly compensated for by a lower My The most interesting case
is the Little Blue Heron. This species may nest in the sun and, being
dark, increase Its thermal load. This increased thermal load at
T > (most of the day and most days of the nesting season) probably
reverses the thermal gradient from skin surface to feather surface, so
that net heat flow is into the body (Lustick et at., 1970). The
greatly reduced It in Little Blue Herons is probably an adaptation to
nesting In an environment with high Insolation. This is not in itself
unique and has been reported for Poor-wills (Pnalaenoptilus nuttalli)
by Bartholomew et at. (1962).
The interrelationship between dark color and metabolism has
been investigated before, but generally in cooler environments. it
can be summarized as follows: 1) birds dyed black (Hamilton
and Heppner 1367) and dark birds (Lustick, 1989) show a 23
26 percent reduction in the rate of metabolism when they are exposed
to (artificial) solar radiation at temperatures below thermoneutraltty.


RATE OF METABOLISM (cc02/g-hr)
(Do) q


77
Table 10.
Cooling constants
1 n nes 11 i ng herons.
Period
Spec ies
Age
1
1 1
1 1 1
Louisiana Heron
2
59-57
4
14.52
4
19-07
5
32.04
6
25-79
10c
16.12
11
16.52
14
2.66
10.79
Snowy Egret
2
23-95
5b
17.20
6
1 1.20
13-15
9
5.12
10.67
17.80
13
2.77
7.80/6.07
14.80
Cattle Egret
3
23.47
4
23.29
8
7.10
19-24
35.84
9
9-75
17.58
9
6.48
21 .30
13
1 .20
9.40
18.18
14
2.10
9.52
19-86
Little Blue He ron
3
23.11
5
19.38
10
6.58
10.98
21.87
aTaken from slopes
of Figures
8-11; numbers
in body of table
are xl0 ^.
Raised in lab; body weight corresponds to 2-day-old chick.
CDead.


CHAPTER 1
INTRODUCTION
Herons (Ardeformes, Ardeidae) are nonpasserine birds with a
wide range of size and distribution. Many of them breed in Florida;
among these are four small species all with relatively similar nesting
habits. Three of these species are probably congeneric (contra AOU
Checklist, 1957); the fourth is a recent Old World arrival.
The Louisiana Heron {Egrelta [-Hyrfranassa] tricolor) is probably
slightly larger than the other species. It has a bicolored pattern,
being a dark gray dorsally and white ventrally. The Little Blue Heron
[Egretta [- Florida] caerulea) is the darkest bird, having a slate blue
body plumage and rust-colored head and neck as an adult. The Little
Blue Heron is unique among these four herons in having a marked
difference between adult and Juvenal plumage. Immatures have a white
plumage which toward the end of their first year gradually molts to the
adult plumage. Snowy Egrets {Egi-etta ihulo. thula) have a completely
white plumage. The. Old World Cattle Egret (Bubuleus ibis) probably arrived
from Africa on the northeast coast of South America in the nineteeth
century, was first seen in Florida ir 19^1 and only began breeding in
Florida in 1953 (Palmer, 1562}. The Cattle Egret; is basically white
with patches of buff color cn the head, back, and breast during the
breeding season.
The inclusion of Little Blue Herons and Louisiana Herons with
Snowy Egrets in the genus Egretta has only recently been proposed
I


were closer to land (farther from open water), mainly in buttonbush
swamp, and averaged 8 feet above the ground. Palmer (1982) states
that Little Blue Heron nests tend to be grouped apart in mixed species
rookeries. Bent (1926) found Louisiana Herons nesting on the ground in
one rookery and as high as 15 feet in another. He noted that these
herons occupied the center portion of mixed species heronries.
However, Palmer (1963) says Louisiana Herons may also group around the
periphery of mixed species colonies. Bent (1926) discovered Snowy Egrets
nesting in open areas on the interior parts of an island (away from open
water) whereas Mean ley (1955) found them next to open water in a lake
colony. Palmer (1962) suggested that Snowy Egrets can nest over 30
feet above ground, although 510 feet was more common. The influence of
insolation may not clarify this picture entirely, but it is an important
step in this direction.
Solar radiation was measured at nests on Biven's Arm and Riomar
island. Table 1 shows the solar radiation incident at the nests of all
four species of herons; an F test showed that there were significant
differences among the herons (p < 0.05). Observations in the field suggeste
that Louisiana Herons nested selectively in the shade. The other herons
appeared to nest without any real reference to insolation. The data in
Table 1 corroborate this: Louisiana Heron nests receive the lowest mean
solar radiation input. Furthermore, botn the upper and lower limits of
the Louisiana Heron's range of insolation are lower than those of
the other three species. Nonetheless, chore is a broad overlap in the
ranges of solar input at the nests of all four species. In order to
clearly show the difference between Louisiana Herons and the others,


95
Carr, M. H. 1942. The breeding habits, embryology and larval develop
ment of the large-mouthed black bass in Florida. Proc. New
England Zoological Club 20: 43~77-
Cowan, P. J. 1972. The contrast and colouration of sea-birds: an
experimental approach. Ibis 114: 390-393-
Craik, K. J. W. 1944. White plumage of sea-birds. Nature 153: 288.
Curry-Lindahl K. 1971- Systematic relationships in Herons (Ardeidae),
based on comparative studies of behaviour and ecology. Ostrich
Suppl. 9: 53-70.
Darwin, C. 1896. The Descent of Man and Selection in Relation to
Sex, 2nd. ed. D. Appleton and Company, New York. 688 pp.
Dawson, V/. R., and J. W. Hudson. 1970. Birds. In Comparative Physiology
of Thermoregulation (G. C. Whittow, ed.), vol. 1, pp. 223310.
Academic Press, New York.
Depocas, F., and J. S. Hart. 1957- Use of the Pauling oxygen analyzer
for measurement of oxygen consumption of animals in open-circuit
systems and in a short-lag, closed-circuit apparatus. J. Appl .
Physiol. 10: 388-392.
Dickerman, R. W., and K. C. Parkes. 1968. Notes on the plumages and
generic status of the Little Blue Heron. Auk 85: 437440.
Drent, R. 1970. Functional aspects of incubation in the Herring Gull.
Behaviour Suppl. 17: 1-132.
Drent, R. 1973- The natural history of incubation. In Breeding Biology
of Birds (D. S. Farner, ed.) pp. 262-311. National Academy of
Sciences, Washington, D.C.
Dunn, E. H. 1975* The timing of endothermy in the development of
altricial birds. Condor 77: 288-293-
Dusi, J. L. 1966. The identification characters of nests, eggs and
nestlings of some herons, ibises and anhingas. Alabama Birdlife
14: 2-8.
Dusi, J. L., and R. L. Dusi. 1970. Nesting success and mortality of
nestlings in a Cattle Egret colony. Wilson Bull. 82: 458-460.
Ellis, H. I., and J. P. Ross, (in Prep.) Field observations of cooling
rates in Galapagos land iguanas (Conolophus suboristatus).
Enger, P. S. 1957- Heat regulation and metabolism in some tropical
mammals and birds. Acta. Phys. Scandinav. 40: 161 166.
Gates, D. M. 1962. Energy Exchange in the Biosphere. Harper and Row,
Publishers, New York. 151 pp.


37
low A, although it is very dark. Lasiewski and Dawson {1967) 'found
D
its to be 99.3 percent of that expected for a.passerine of its
weight.
Metabolism and insolation
In Turkey Vultures and Roadrunners, basking makes hypothermic
birds normothermic. But Little Blue Herons, Louisiana Herons,
Snowy Egrets, and Cattle Egrets all have the same high T^ of about
40C. Insolation at high can only make these birds hyperthermic;
their Tjj rises as they store heat. But because lethal T^ for most
birds is around k5~k6C, there exists only a small margin for error in
hyperthermic birds. The rate of change in body temperature (AJ^/At)' of a
bird which is receiving solar radiation in a hot environment will be
a function of internal heat production and external heat. The external
heat load will be transmitted through the thermal gradient between
feather surface and skin. This can be expressed as conduction
dQ
dt
(7)
. where Q is heat; t is time; A is surface area; T is temperature; X is
distance, and X is a constant. The distance between feather surface
and skin may be considered constant, and equation (7) becomes
^ = Cn(T- T )
dt If s
(8)
where T~ is feather surface temperature; T is skin temperature, and
J s
C-, is a constant KA/X. Therefore, the rate of change in T-, can be
expressed
A T, M + CJT T )
& If s
H KW
(9)


5
maps from 1967 do not show them on the southwest shore of the lake,
The northeast heronry, on the other hand, is not clearly separated from
the shoreline. A swamp of partly submerged vegetation, narrow water
ways, and floating islands spans the few hundred meters between pasture
and open water. The heronry occupies that part of the swamp closest to
the open water, rarely penetrating more than 75 meters from it.
The four species of herons studied on Siven's Arm nested primarily
in red maple (Acer rubrum) and elder (Sambucas simpsonii) which they
shared with Anhingas (Anhinga ankinga) and, in most years, White ibis
(Eudoeimis albus). Some buttonbush (Cephalanthus occidentaiia) was
intermixed with the red maple and elder in 1971$ but virtually all
of it died that year, and it only began to reappear in 197k. Floating
cut-grass {Leersia hexandra) was abundant at the margins of the
heronries and also scattered throughout most of the northeast heronry
along or in the waterways. Pennywort (Eydroaotyle spp.) existed in
spots at the water's edge at the heronry sites. By 1973 and especially
197^, very large growths of water hyacinth (Eiohhoz^vla. crassipes)
covered the water surface adjacent to the heronries.
Several other avian species nested within the heronries:
Green Herons [Buiorid.es viresaen.z), Common Gallnulas (Gallnula
okloropus) ; Purple Gall inules {Vovphyvvda vizvtinioa) ; and Least
Bitterns {.Lxobryckus exilis); whose numbers increased dramatically in
1875 perhaps due to the expansion of water hyacinths. A very few
American Coots {Flica americana) and Black-crowned Night Herons
{Nyatioorx nyciicorax) were occasionally seen in the heronry, but
without evidence of breeding, at least during the spring and early


Table 9. Body temperatures of young herons at the nest.
Spec ies
Nib
N2C
Ta Range3
a n d
Age Range
Louisiana Heron
38.9 +
0.3
23
19
20.5 36.4
0-9
Snowy Egret
38.9 +
0.1
3
3
(31-5)
0-2
Cattle Egret
38.9 +
0.2
16
16
26.0 32.6
1-14
Little Blue Heron
38.5 +
0.6
7
5
27.0 35-0
0-5
aMeasured in C.
b_ .
Total measurements,
c
Number of different individuals measured,
d
Measured in days, where the first day of life is age 0.


i
I
TIME (min)


61
may still spend a great deal of time at the nest, particularly in the
middle of the day when Tis maximum. Attentiveness does gradually
diminish with time however.
During the period of attentiveness, the parents behave in such a
way as to ameliorate the thermal environment. This behavior changes
as the nestlings grow older. The adult usually broods the nestlings
for about one week after hatching. At first this may serve the
additional purpose of incubating any remaining eggs in the nest. But
even after the last young is hatched, the adult will frequently brood
its nestlings. This behavior, which shields nestlings from cool Tor
cooling breezes, was usually seen in the morning or evening hours.
Nestlings as old as 5 or 6 days were often brooded in this way. Occa
sionally older chicks were similarly brooded; for example, a Snowy Egret
(SE 3) parent lay on its 13-day-old nestling under a cloudy sky when
Ta = 32C or less on May 31, 1975. It is possible that this parental
behavior was normal at night. On July 7, 197^, at 1600 hours an adult
Cattle Egret (CE106) brooded four young aged k~7 days; another Cattle
Egret (CE 109) was seen to gently lie down on the 12-day-old nestling
which had been standing between its legs; a third Cattle Egret (unnumbered)
stood hunched over with its large 18 day nestling between its legs.
No evidence of shivering was noted for any of the species as
reported for Cattle Egrets by Blaker (1969) and Hudson et at. (197^),
primarily because most observations were made from a distance, usually
with the aid of binoculars. However, the parental behavior of brooding
the very young (< 6 days) nestlings would probably compensate for any
lack of the ability to shiver in these birds. Interestingly, Hudson


80
Herons achieve homeothermy more slowly than the other species. At
present, the poor fit of the Louisiana Heron data to their slope cannot
be explained. However, it results in such anomalies as the two 4-day-old
chicks having a cooling constant comparable to an 11-day-old nestling.
Cattle Egrets show the fastest development of homeothermy, followed
by Snowy Egrets. Their slopes from Figure 11 are -0.261 and -0.226
respectively. Hudson et at, (1974) indicate that endothermy is achieved
in the 10-12 day-old class of Cattle Egrets.
Two nestlings were removed from their nests at 1 day and taken to
the laboratory. One was a Louisiana Heron, the other a Snowy Egret.
Growth was stunted in these birds so their weight at 5 days, when cooling
curves were made, resemb1ed that of nestlings only 2 days old. Neverthe
less, the Snowy Egret showed a cooling constant in keeping with its age.
The 5_day-old Louisiana Heron is difficult to evaluate due to the scatter
of data in that age group.
The ability of a brood to become homeothermic before any individual
in it could regulate has been reviewed by Dunn (1975). An extreme case
was cited by Yarbrough (1970) where a brood of 4-5 Gray-crowned Rosy
Finches (Leuaosticte tephrocotis gviseomicha) could regulate their
temperature 6 days before any individual could. The huddling in herons
as described above is probably a mechanism to allow collective homeothermy
at relatively low T(<_ 30C) before the individual nestlings were able
to thermoregulate. As might be expected, older nestlings (> 10 days)
were not observed huddling. Unfortunately, cooling curves were not
developed for nestling broods in this study. But McManus and Singer
(1975) working with the Mongolian gerbil (Meriones unguiculatus)


A
10 -
0
A
x SNOWY EGRET
o CATTLE EGRET
A LOUISIANA HERON (sun)
A LOUISIANA HERON (shade)
LITTLE BLUE HERON
20
SoA (l-r)
T
30
40
Tr~
O


Figure 7- Attentive time comparison between Little Blue Herons
and Cattle Egrets in sun and shade.


Tab le 1.
soiar
radiation0 on
heron nests.
Louisiana Heron
Snowy Egret
Cattle.Egret
Little Slue Heron
Range
0.046 1.279
0.136 1.306
0.112 1.473
0.08i 1.391
x +_ s e.
0.505 + 0.070
0.595 + 0.155
0.761 +0.050
0.645 + 0.064
N
33
10
84
51
% above I.
?00a 5.)
20.0
27.4
19.6
aHeasured
r.>
in Langleys/min -- cal/cm min.


16-
sky by building their nests just below a large branch (Calder, 1970-
Roadrunners (Geoaoocyx califomian) often build nests which are partly
exposed to the sun but which have bands of shade to provide nestlings with
a choice of thermal environments (Ohmsrt, 1373)- Cactus Wrens (Campylo-
rJujnchus brmnneicapillus) build closed nests whose entrances face away
from the wind in the early cooler part of the breeding season but face
them during the hot part of the season (Ricklefs and Hainsworth, 1969)-
The effect of solar radiation on an animal can be described by the
term SoA[l t) [modified from Gates (196)] where So is solar radiation,
A is the animals surface area, and v is its reflectance. Because the
Louisiana Heron, Snowy Egret, Cattle Egret, and Little Blue Heron are all
about the same size, A can be considered the same for all of them. So is
also the same, because they all live together. So any differences in the
effects of solar radiation on these birds will be a consequence of their re
flectances. Figure 1 shows the percent reflectances for all these species
as 3 function of wavelength as measured at the dorsal surface. The Little
Slue Heron and Louisiana Heron are markedly darker than the Snowy Egret and
Catite Egret. The reflectance curves for the Louisiana Heron and Snowy
Egret are probably slightly lower than norma! because the specimens used
had a duller plumage than is normally observed in the breeding season. How
ever, it is doubtful that the curves would change very much with brighter
plumage. Snowy Egrets and Cattle Egrets show a reflectance peak at the
long visible wavelengths. Interesting!/, the Little Blue Heron white
juvenai plumage reflects more radiation over most wavelengths, including
ultraviolet and near-infrared. Whether this is characteristic of
juvgnal plumages in the white Snowy Egret and Cattle Egret is unknown.
Reflectance was not measured for the juvenal plumage of Louisiana Herons,


Figure 12. Rates of homeothermic development in herons.


21
i970), thus determining (7^ 7^), the zone of thermoneutrality. A
consideration of the determination of (7, TQ) will follow iater.
Both metabolism and conductance are weight~dependent functions.
Lasiewski and Dawson (1967) showed of nonpasserines to have the
following relationship
Mh 78.3W
o
d0.723
(4)
where is in kcal/day and W is weight in kg. The form of equation (4)
can be modified (McNab, 1974) to
Mj/W = 4.6rv'?'8
(5)
(6)
where is in ccO^/g-hr, assuming 4.8 cal/ccO^ (Brody, 1945), and
W is in g. Conductance in birds is related to weight (Lasiewski et
al.j 1967) as
c/w = 0.
where C/W is in cc0o/g*hrC.
4.
Figures 2-,5 show the metabolism of Louisiana Herons, Snowy Egrets,
Cattle egrets, and Little Blue Herons, respectively, as a function of
ambient temperature. Minimum conductance is shown extrapolated through
Tfo at M ~ 0. Table 2 summarizes these results. Table 3 compares the
observed basal metabolism and minimum conductance with that expected
based on equations (5) and (6) respectively.
Metabolism
Resting metabolism was measured during daylight hours for adults
of all four species of herons. Daytime metabolic measurements are a
better estimate of heron energetics in the field when the birds
experience thermal stress. A few nighttime measurements of metabolism


igure 3-
Snowy Egret metabolism.


63
1.4-2.9 or more times their metabolic heat production by evaporative
water loss at high T (44-45C). Most of this evaporative water loss
came from guiar fluttering and panting, but some was presumed to come
from excrement smeared on the abdomen of young nestlings before their
plumage was fully developed. I have observed excrement on the abdomens
of nestlings of all the heron species investigated. Whether or not high
T stimulates excretion in the nest is speculative. However, one
a
nestling Cattle Egret raised in the lab was capable of excreting outside
the "nest" after about 10 days. Ricklefs and Hainsworth (1969) showed
that Cactus Wrens nesting in the desert left more fecal sacs in the nest
as Ta increased through the breeding season. This resulted in a lowering
of nest temperatures; effects on nestling T-, were not discussed.
The interaction of nestling and parental behavior may often be
complex and results, presumably, in the stabilizing of at a high level
in the nestlings. Two examples should illustrate these behaviors. Two
neighboring Cattle Egret nests were observed July 3, 1974 between 1030 and
1200 hours. At the start of this period, the wind was moderate to brisk,
and Ta < 28 under a cloudy sky. The parent at nest CE-105 lay on two
nestlings, aged 3 and 4 days, until about 1100 hours. At that time, the
sun broke through, and the wind calmed. Tin the sun was not determined
at this time, but when the sun reappeared an hour later under similar
conditions, T1 = 36.0C. The parent then stood up and shaded its two
nestlings. At 1120, clouds obscured the sun again, and the adult lay
down on the nestling once again. In nest CE-106, there were four
nestlings which included individuals older than the nestlings of CE-105.
The parent of CE-106 was somewhat less attentive, so thermoregulation


86
parent has left the nest for an hour. Even at T= 28.9C, the older
two siblings do not huddle. From Figure 11, the cooling constant of the
youngest bird is 0.0129. After 60 min, T^ will have dropped to 33-5C.
The absorption of solar radiation- in both the examples above could
prevent the large drop of T^. This is probably a function of the dark
skin. It is interesting that the small pink-skinned chick of White
Ibises hatches out completely covered in dense black down. Bartholomew
and Dawson (195^) indicated that down may protect young birds, including
Great Blue Herons (Ardea heroidas), from solar radiation. Although that
must certainly be true once internal heat production is relatively high
and the surface/volume ratio is relatively low, very young chicks may
face prob 1ems of heat loss rather than heat gain.
Linsdale (1936:117) reported a correlation of nestling plumage
(especially down) with insolation. Looking at 15 Great Basin icterids
and fringi11 ids, he noted that "birds which nest in exposed situations
and which live in hot regions have pale or pallid nestling plumages
and nest linings which reflect" solar radiation.
The reason why dark Little Blue Herons have young with white
plumage whereas dark Louisiana Herons have young with dark plumage has
to do with nesting patterns and the transition from a problem of heat loss
to one of heat gain. If adults leave their young unattended once the
oldest nestling is about 12.6 days old, younger siblings will not yet
be homeothermic. Even the older siblings will not be totally independent
of their thermal environment. But the younger siblings will not be
capable of leaving the nest very readily. If left alone under high
solar intensity at high T the chicks might not survive. Their white


TABLE OF CONTENTS
ACKNOWLEDGMENTS. . ii
ABSTRACT ........ . v
CHAPTER
1 INTRODUCTION I
2 METHODS AND MATERIALS. . k
Study Areas h
field Techniques ............... 7
Laboratory Techniques. 9
Statistical Methods II
3 NEST SITES AND SOLAR RADIATION 12
A ADULT METABOLISM 20
Metabolism '.
Conductance. .....
Body Temperature . .
Energetic Implications
5 THERMOREGULATION AT THE NEST 50
Incubation 52
Behavioral Temperature Regulation. ....... 55
Development of Homeothermy .......... 66
Color in Nestling Herons 83
6 THE SIGNIFICANCE OF COLOR IN HERONS 88
7 CONCLUSIONS. .... ...... 92
REFERENCES 9*
BIOGRAPHICAL SKETCH. oi
z l
41
46
48
¡ V


53
cloudy days. To best handle the data, an arbitrary decision was made to
consider all nests more than 50 percent in the sun as "sunny" and nests
with 50 percent or less sun (due to clouds or natural shade) as "shady."
A total of 35-9 hours were spent watching seven Little Blue Heron nests
and four Cattle Egret nests for 60 min or longer periods at randomly
selected times between 1030 and 1730 hours under a variety of environ
mental conditions. Attentiveness (percent time on the eggs) was con
sidered as a function of ambient temperature. The results are presented
in Figure 7- As T increased, attentiveness tended to decrease for both
species. Shady nests were about the same for both, although Cattle
Egrets appeared to be slightly more attentive. At sunny nests, however,
although attentiveness continued to fall with increasing T, Little
Blue Herons spent noticeably less time on the nest than did Cattle Egrets.
Lines connecting two points in Figure 7 represent pairs of measurements
made simultaneously. Here environmental conditions such as wind, humidity,
and cloud cover were identical; only the degree of cover from the sun
varied. All pairs involved one sunny nest and one shady nest. Four of
five lines connecting pairs of Cattle Egret nests are mostly horizontal
reflecting the expected difference in Tbetween a sunny and shady nest.
However, the lines connecting pairs of Little Blue Heron nests have a
considerable vertical component as well. Little Blue Herons were behaving
in the sun as i f T were even higher than it was. This must be a conse
quence of dark plumage increasing the thermal load on a bird, probably
by reversing its skin to feather surface thermal gradient (see Chapter k).
The general relationship of decreasing attentiveness with increasing
T has been noted before for Cattle Egrets (Lancaster, 1970), White Ibis


In (Tb-Ta)
20-
10-
2-
3 0
2.5-
2.0-
1.5-
1.0-
10
i r~
1 T"
i r~
30
50
70
90
110
130
150
170
190
TIME (min)


33
where M is rate of metabolism, K is specific heat, and W is
weight.
I he net heat gain to the bird at high T is
CJT, T ) = SoA(l r) C0(T c ~ T )
i J s 2 f a
00)
where C^(Tj. T^) is the conductive heat gain from the feathers,
SoA(l v) is the solar radiation striking the feathers, and Cn(T T )
2 j- a
is the convective heat loss from the feathers, Cn is a combined constant

"h g
for convective heat loss (all other symbols are as previously
defined). Equation (10) can be rearranged so that
qi _
f
SqA (1 ~ r) + CX + CnT
Is 2 a
C1 + c2
00
Substituting equation (li) into equation (9) for T
-p*
M + C
A T,
b
At
1
SoA (1 r) + CJT + Cr,T
1 s a
V T
KW
(12)
Assuming T^ and Ta to be constants, equation (12) simplifies to
AT, M + K- [SoA (1 v) ] + Kc
b_ ~ I
A t KW
03)
C,
wnere if = 77
C, T + CXJ
i 7t Is 1 a a
and K =
\s + Co
CAT .
J s
2 "1
It is now apparent that the rate of change of T% isa function of
D
both rate of metabolism and solar input. Figure 6 shows the relation
between expressed as a percent of that expected bv weight (see Table
3) and solar input, SoA (2 r). SoA (1 r-A was calculated using the


*3
T are essentially behavioral (see below), (TV Tt) of Little Blue
a o L
Herons (12.8 +_0,5C) is significantly lower than that of the other-
herons (15.9 + 0.5C) using Student's t-test (P < 0.05).
Determination of Thermal Conductance
Conductance is the coefficient of heat loss in the Newtonian model
of equation (1). In an endotherm whose total response to T below TV is
CL X/
chemical regulation, C is the slope of the regression of metabolism, on
decreasing Tf.- in these cases C extrapolates to T^ in accordance with
equation (I). This is rarely the case in birds, however. Most birds
show a mixing of chemical and physical regulation at T < 2^. This
manifests itself as a steadily decreasing C as T decreases (Schmidt-
NSelsen, 1975: 323)- In this case the regression of metabolism on T
extrapolates to a figure in excess of The slope of this regression
if presented as C will be an underestimate. Contrary to King (1964)
this does not invalidate Newton's law of cooling as a model (Scholander
et at,, i950)i it simply reaffirms that birds mix physical responses with
chemical responses to low T In these cases there may be a family of
slopes which decrease as T decreases and which converge at TV iGf-
Figures 2-4). Minimum thermal conductance is that conductance which
represents purely chemical responses to decreased T ; where mixing of
CL
responses occurs, it is found at the lowest T values. This minimum
conductance is the C of equation (1).
Although C cannot be equivalent to the slope of a line which does
not. extrapolate to it can be. calculated in several ways. in this
oarer C was considered to be the lowest slope of the family of slopes


Table h. Comparative metabolism of some hot environment
nonpasserines with dark plumage and/or skin.
Spec!es
N
Vr ?'a
Source
Fgretta ( Florida) oaerpla
Little B!ue Heron)
2
66.0
Present study.
Geooocayx californianus
{Roadrunner)
9(7)
97
Calder and Schmidt-Ntel sen,
Geococcyx caZifornianus
6
82.2
Ohmart and Lasiewski, 1971
Catkavtes aura
(Turkey Vulture)
2
70.if
Enger, 1957.
Covagyps atraius
(Black Vulture)
2
79-6
Enger, 1957.
Fregata magnificons
(Mean ifs ci en t Frigatebird)
4
69.4
Enger5 1957.
'See Table 3
1376.
Co
O''


Table 3*
Energetics of herons.
Species
W
Mb/W
Mb%b C/W
c%c
Mp/C
(Mb/C)ed
Cl
F-
Tb t Louisiana Heron
303.3
0.992
107 0.0590
129
16.8
20.2
0.83
40.0 25,8f
Snowy Egret
314.0
0.784
85 0.0500
1 10
¡5-7
20.3
0.77
40.3 26.2
Cattle Egret
293.3
0,738
85.5 0.0500
107.5
16.0
20. 1
0.80
40.3 26.6
Little Blue Heron
290.4
0.620
66 0.0462
98
13.4
19-9
0.67
40.2 27.5
aBased on 2 Indi v
1 duals c
if each spe
c:es, except as
noted.
Symbols
as in Tab!
e 2.
^'Percent of expec
t e d Mi =
r>
(McNab, 197**,
mod ified
from Lasiewski and
Dawson
, 1967).
'Percent of expec
ted C =
0.854"' 51
(Lasiev/ski et
al.y 1967).
Exoected M-, /C
D
4,6 W
r/- 0. 28
0. Stv"0 1
5.41 '/
23
'Ratio of observed M-./C to expected M-./C
O u
Based on LOU 2 only.
W


tures of the herons were taken with a Schultheis thermometer inserted
3~5 cm Into the cloaca, within about a minute of the final metabolic
measurement. All the adult herons measured, except the Cattle Egret,
were raised in captivity and weighed about 20 percent less than wild
herons. A weight difference between wild and captive-reared birds
has also been noted for the Redhead (Aythya americana) by Weller (1957).
Statistical Methods
A variety of statistical tests was used, but all measures of
variance given in the body of this paper are standard errors of the
means, unless otherwise noted.


67
(197*0 who investigated Cattle Egrets. Their study in fact demonstrated
the development of endothermy in Cattle Egrets. That was not possible
in this study since metabolism of nestlings was not measured. As a
result, homeothermy cannot be linked conclusively to a given level of
internal heat production but must instead be defined as the ability of
young birds to maintain a 2^ at least 75 percent of adult T^ at low T
(ca. 20C) for one or two hours (Dunn, 1975). This ability may be due
to increased insulation because of emerging plumage, increased vasomotor
control, reduced surface/volume ratio, better muscular coordination
(e.g., more effective shivering), a.higher rate of metabolism, or a
combination of some or all of these.
Figures 8-11 show nestling cooling curves at different ages for
Louisiana Herons, Snowy Egrets, Cattle Egrets and Little Blue Herons
respectively. The temperature differential (T^ T ) is plotted as its
natural log against time. The resulting slopes are therefore the cool
ing constants (a) with units of min \ Generally, the older the nestling,
the slower it cools. Therefore the cooling constants get progressively
smaller with increasing age. At least brief periods of homeothermy
are attained by 13 days.
Many of the curves for a given individual show two or more slopes.
This is first seen in nestlings 8 or more days old but one Snowy Egret
showed it at 6 days. A young heron is apparently capable of maintaining
increasing (2^ Tlevels for short periods of time. After about an
hour (Tjj Ta) drops at a rate reminiscent of younger nestlings. This is
suggestive of a drop in metabolic activity. When the final cooling
constants [i.e., the only cooling constants in the younger nestlings)


76
are compared for most species, they are remarkably similar, showing only
a small decrease with age (see Table 10). This residual difference is
probably due to a decreasing surface/volume ratio as well as emerging
plumage. It is likely then that initial cooling constants in older
nestlings reflect a considerable metabolic contribution.
The initial cooling constant is probably of greatest ecological
significance. It represents the cooling a nestling would first undergo
when left alone. Usually the change to a higher a does not occur for at
least hO min. By that time, these nestlings (pearly all branchers by this
age) could change their thermal environment by, for example, moving into
sunlight. Only on cool windy days in the absence of a parent would this
time be exceeded before a suitable environment could be found. Ellis
and Ross (in prep.) demonstrated that cooling constants could be used
to predict the time a poikilotherm (here prehomeothermic birds) had
before its approached T :
ln(Tb Ta)t Mlj
T ) .
a %
(17)
-a
where (T T-, ) is the initial temperature differential.
a d is
The natural log of the initial cooling constant is plotted
against age in Figure 12 to determine differences among the species.
The correlation coefficients, r, for Snowy Egrets, Little Blue Herons,
and Cattle Egrets are 0.99, 0.99, and 0.95 respectively, whereas r
for Louisiana Herons is only 0.8l. The slope (-0.173) for Louisiana
Herons is the lowest, and although It is close to that of Little Blue
Herons (-0.186), most of the Louisiana Heron data lie above their
slope. The Lousiana Heron also showed the poorest level of homeothermy
at the 1314 day age class. This all tends to indicate that Louisiana


41
average reflectance from Figure 1, So ~ 1.43 ca 1/cm'"-min, and
A. W '/2 (because rarely would solar radiation impinge on more
than 50 percent of a bird's surface area).. Figure 6 shows that
Louisiana Herons in the sun have high % M-^ and high SoA(l v)
due to their low r. They thus would heat rapidly, and consequently
remain mostly in the shade. Snowy Egrets and Cattle Egrets have a
slightly depressed % M, but their SoA(l r) is very small due to
their high Little Blue Herons have the lowest % M^ which compensate
for their high SoAil v) that a low r gives them. Little Blue. Herons
can take advantage of a low M-. when in the sun so that their rate of
b
Jb increase is relatively slow.
Conduetanee
Table 2 summarizes the data provided in Figures 2.-5 for minimum
thermal conductance. Table 3 compares these minimum conductances with
the conductances predicted by weight in equation (6). C is equal to
or greater than expected in every case. There is a clear trend in
these values similar to the data for Af, Louisiana Herons have the
b
highest C (123 percent) and nest in the shade. Snowy Egrets and Cattle
Egrets which may nest in the sun but are white have intermediate values
of 110 and 107-5 percent respectively. The dark, often sun-nesting
Little Blue Herons show a C of 38 percent. Dawson and Hudson (1970)
indicate that birds in hot environments tend to have high values of
C for heat dissipation. Yarbrough (¡571) cites data to that, effect.
What then is the significance of a relatively low Cl it is doubt
ful that these herons can use it to much advantage to insulate themselves


BIOGRAPHICAL SKETCH
Hugh Irl Ellis was born May 29, 1944 in Burbank, California.
He attended school in Southern California and began college at the
University of California at Berkeley in 1962. He received his A.B.
in Zoology from there in 1967- Subsequently he was enrolled at
California State University at Northridge (then San Fernando Valley
State College), where he received an M.S. in Biology, from 1967 to
1970. In 1970, Mr. Ellis enrolled in graduate school at the University
of Florida in pursuit of a Ph.D. in Zoology, which is to be awarded
in June, 1976. His main fields of research interest are physiological
and behavioral ecology. In 1974 Hr. Ellis was named Teacher-of-the-Year
by the Graduate School. In 1975 he received a Chapman Memorial Fund
grant to help support his research. Mr. Ellis is a member of the
Ecological Society of America, the American Association for the
Advancement of Science and the American Ornithologists' Union.
101


Figure 11. Little Blue Heron cooling curves.


Figure 4.
Cattle Egret metabolism.


98
Meanley, B. 1955- A nesting study of the Little Blue Heron in eastern
Arkansas. Wilson Bull. 67: 84-99.
Heyerriecks, A. J. i960. Comparative breeding behavior of four species
of North American herons. Publ. Nuttall Ornithological Club, No.
2. 158 pp.
Murtn, R. K. 1971- Polymorphism in Ardeidae. Ibis 113: 97"99.
Nordlie, F. G. 1976. Plankton communities of three central Florida
lakes. Hydrobiologia 48: 6 5-78 -
Ohmart, R. D. 1973- Observations on the breeding adaptations of the
Roadrunner. Condor 75: 140-149.
Ohmart, R. D., and R. C. Lasiewski. 1971- Roadrunners: energy con
servation by hypothermia and absorption of sunlight. Science
172: 67-69.
Orr, Y. 1970. Temperature measurements at the nest of the Desert
Lark (Ammomanes desevti deserti). Condor 72: 476-478.
Palmer, R. S. 1962. The Handbook of North American Birds, vol. 1.
Yale Univ. Press, New Haven. 567 pp.
Recher, H. F. 1972. Colour dimorphism and the ecology of herons.
Ibis 114: 552-555.
Recher, H. F., and J. A. Recher. 1969. Comparative foraging
efficiency of adult and immature little blue herons (Florida
aaerulea). Anim. Behav. 17: 320-322.
Ricklefs, R. E. 1967- A graphical method of fitting equations to
growth curves. Ecology 48: 978-983-
Ricklefs, R. E. 1968. Patterns of growth in birds. Ibis 110: 419-451.
Ricklefs, R. E., and F. R. Hainsworth. 1969- Temperature regulation
in nestling Cactus Wrens: the nest environment. Condor 71:
32-37.
Rudegeair, T. J., Jr. 1975- The reproductive behavior and ecology of
the White Ibis (Eudooimus albus). Ph.D. Dissertation, Depart
ment of Zoology, University of Florida.
Schmidt~Nie1 sen, K. 1975- Animal Physiology. Cambridge Univ. Press,
New York 699 pp.
Scholander, P. F., R. Hock, V. Walters, F. Johnson, and L. Irving.
1950. Heat regulation in some arctic and tropical mammals and
birds. Biol. Bul 1 99: 237-258.


87
plumage (see Figure 1 for Little Blue Heron fledgling) helps protect
them by reflecting much of the solar radiation. The Louisiana Heron
generally nests in the shade. Presumably nesting in the sun would
require more parental attention to keep their older dark nestlings from
overheating. One Louisiana Heron nest was found in the sun in 197^- The
adults continued attentive behavior (mostly shading one nestling) for
23 days. This is longer than observed for any other nest of any of the
four species. Such intense parental care must necessarily reduce the
amount of food gathering possible and so lower the number of young
fledged from the nest.


COOLING CONSTANT (MIN'1)
o.oi-
0.001-
^1
V-O


t-~ test.
It appears that a high 2^ is being selected for.
In the equation
- d1 (- T^), Ty is the dependent variable. Little Blue Herons then
show a different temperature response to hot environments than do
Turkey Vultures (Heath, 1362) and Rcadrunners (Ohmart and Lasiewski,
1971) both of which bask to compensate for hypothermia. Little Blue
Herons, which are normothermic and do not bask, adjust T^. As stated
earlier, the differences probably are cue to the. unpredictable food
supply cf Turkey Vultures and Rcadrunners,
Energetic 1mp11 cations
Yarbrough (197) has argued that tropical or desert {i.e. warm
habitat) birds show a low F value [his (M./C) ] and have a low (T, T0).
The. F values in Table 6 are in fact low with the Little Blue Herons
(F 0.67^ + O.O^O) being significantly lower than the pooled F of the
other species (0.803 + 0.03*0 using a -test (P < 0.09). Table 6 also
shows that (2V T^) is lower than the expected ratio (Mh/C)would
predict.
Siegfried (I969) measured "aviary-existence energy level" as
100 kca/day for a 383 g Cattle Egret at 16-19C. This is equivalent to
2.27 cc0/qhr where average T 175 C- Siegfried showed that this was
' o.
about triple standarc metabolism (AT.);, actually it is 2.61 times M%
o o
[calculated as predicted by Lasiewski and Dawson (2967) in equation. (5)
above]. Figure k shows that a Cattle Egret uses an average of 1.21
0o/g"hr at !?
g 0 C.
Hence aviary
ex!ste
nee energy i
s 1.88 ti mes
sting metabo21
ism at
tne lo-I3 [y 5
-ange.
Th i s va1ue
is a more useful
tmate than Si
ieg f r
ed!s. If one
cou 1 d
generalize 1
:c all ambient


^9
temperatures, existence energy is about double resting rates, a
figure which could be used as a starting point for predicting energy
budgets for these birds.


in one Cattle Egret (CE1) showed conductance unchanged, but was
2k percent lower than In the daytime. This is in close agreement with
the estimate of Aschoff and Pohl (1970) that nighttime is 25 percent
lower than daytime AL. There will be no discussion here of whether it
is in fact proper to use the term "basal11 metabolism in daytime measure
ments of diurnal birds. This distinction between daytime and nighttime
measurements cf metabolism is clouded because 1) the time of day is not
always reported and 2) there is a tendency to avoid dealing with the
distinction (cf. Lasiewski and Dawson, 1967)- it should be noted, how
ever, that the low values of reported in this paper as would
have been even lower had they been measured at night.
Table 3 shows that M^ for the smaller individual of a species pair
is always significantly higher than for the larger individual (P < 0.05)
using Student's t distribution (Simpson et al., I960). This should be
expected from the weight relationship in equation (5).
Table 3 also shows that the Louisiana Herons were the only species
whose tip exceeded that predicted by equation (5) (x = 107 percent of
expected). Both Louisiana Herons seemed more active or "tense" in the
metabolic chamber than any of the other birds. Thus, it is possible
that; observed metabolism is a slight overestimate. Nevertheless, this
species still appears to have the highest Mu. Louisiana Herons, which
almost always nest in the shade, can be compared to the three other
species which nest without reference to the sun. The average Snowy
Egret was 85 percent of expected. Although the range was large
(77 and 93 percent), the average was quite close to the 85.5 percent-
average of the Cattle Egrets (whose range, was small). Both of these


90
(as discussed by Johnson and Brush, 1972) which promotes polymorphism
in predators by reducing the ability of prey to generalize a single
search image of a predator, could be occurring. This hypothesis would
probably be easily tested experimentally.
Meyerriecks (1960) considered white color in herons to be con
spicuous in order to facilitate social interactions. He noted that
the most social herons are white. Recher (1972) stated that the
white juvenal plumage of Little Blue Herons may be related to their
post-breeding dispersal and the tendency of juveniles to feed in flocks
at this time.
Recher (1972) noted a correlation between color polymorphism,
coastal (actually, open) environments, and tropical latitude. It is
true that most polymorphism in herons is confined to low latitudes.
Recher stated that in hot exposed environments,"white-coloured herons. .
may be better able to regulate their body temperatures than their
dark siblings" while feeding. Very few workers have looked to see
whether white herons even normally hunt in direct sunlight during the
heat of the day. Siegfried (1971b) found that feeding activity in
Cattle Egrets is reduced during that part of the day. However, I have
shown that white plumage certainly confers a degree of protection from
insolation among nesting Cattle Egrets and Snowy Egrets and nestling
Little Blue Herons. Meyerriecks (i960) rejected the notion that white
color is useful in temperature regulation, because he has seen Little
Blue Herons and Green Herons remain in the sun for hours. The basal
metabolism of Green Herons is not known, but M^ for Little Blue Herons