Group Title: influence of distribution and ecology on the thermoregulation of small birds
Title: The Influence of distribution and ecology on the thermoregulation of small birds
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Title: The Influence of distribution and ecology on the thermoregulation of small birds
Physical Description: ix, 69 leaves : ill. ; 28 cm.
Language: English
Creator: Yarbrough, Charles Gerald, 1939-
Publication Date: 1970
Copyright Date: 1970
Subject: Body temperature   ( lcsh )
Animal heat   ( lcsh )
Birds   ( lcsh )
Ecology   ( lcsh )
Zoology thesis Ph. D
Dissertations, Academic -- Zoology -- UF
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
Thesis: Thesis--University of Florida, 1970.
Bibliography: Bibliography: leaves 64-67.
Additional Physical Form: Also available on World Wide Web
General Note: Manuscript copy.
General Note: Vita.
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Bibliographic ID: UF00097750
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: alephbibnum - 000554289
oclc - 13385420
notis - ACX9123


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I have profited from many discussions with David W.

Johnston and Brian K. McNab. Dr. McNab also provided the

data on the Black-throated Trogon. David Niles and S. A.

Rohwer of the University of Kansas kindly obtained the

Harris' Sparrows for me. The equipment and facilities of

the University of Florida were used throughout the study.

The work was done while in tenure of a National Science

Foundation Traineeship. I wish to credit my wife, Hazel,

for her constant encouragement.


Acknowledgements ii

List of Tables iv

List of Figures v

Abstract viii

Introduction 1

Materials and Methods 5

Results 7

Discussion 28

Heat Loss and Heat Gain 28

Metabolism 30

Conductance 34

The Physical Model 36

Determination of the Level of Tb 39

Body Size Effects 44

Precision of Tb Regulation 46

Ecological and Distributional Relationships 51

Conclusions and Summary 56

Appendix A Symbols and Expressions Employed 59

in This Paper

Appendix B Scientific and Common Names for 61

Species Discussed in This Study

References 64

Biographical Sketch 68



Parameters of the energetic of Tb

regulation in some small birds

Energetic parameters of small birds

selected from the literature

Table 1

Table 2


Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Responses of body temperature and oxygen

consumption to ambient temperature in

Spizella passerina and Zonotrichia albicoZlis.

Responses of body temperature and oxygen

consumption to ambient temperature in

Passerculus sandwichensis and Ammodramus


Responses of body temperature and oxygen 1

consumption to ambient temperature in

Melospiza melodia and Melospiza georgiana.

Responses of body temperature and oxygen 1

consumption to ambient temperature in

Pooecetes gramineus and Passerella iliaca.

Responses of body temperature and oxygen 1

consumption to ambient temperature in

Zonotrichia querula and Zonotrichia


Responses of body temperature and oxygen 1

consumption to ambient temperature in

Parula americana and Vermivora pinus.

Responses of body temperature and oxygen 1

consumption to ambient temperature in

Dendroica pinus, Vermivora celata, and

Mniotilta varia.

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

Figure 15

Figure 16

Responses of body temperature and oxygen

consumption to ambient temperature in

Dendroica coronata and Protonotaria citrea.

Responses of body temperature and oxygen

consumption to ambient temperature in

Wilsonia citrina and Dendroica dominica.

Responses of body temperature and oxygen

consumption to ambient temperature in

Seiurus noveboracensis and Seiurus auro-


Responses of body temperature and oxygen

consumption to ambient temperature in

Dendroica palmarum and Trogon rufus.

Responses of body temperature and oxygen

consumption to ambient temperature in

Sayornis phoebe and Geothlypis trichas.

Responses of body temperature and oxygen

consumption to ambient temperature in

Empidonax virescens and Contopus virens.

Responses of body temperature and oxygen

consumption to ambient temperature in

Myiarchus crinitus and Tyrannus tyrannus.

The relation of basal metabolic rate to

body weight in some small birds.

The relation of thermal conductance to

body weight in some small birds.

Figure 17

Figure 18

Figure 19

Figure 20

Figure 21

Figure 22

The relation of the observed thermo- 38

regulatory quotient (Mb/C)o to body

weight in some small birds.

A plot showing the effect of body weight 38

on thermoregulation data taker 'from birds

used in this study and from the literature.

The relation of the extent of the thermal 41

buffer to the thermoregulatory quotient

expected from body size alone.

A plot of the extent to which body temper- 48

ature is buffered, as a function of the

combined ratios (Mb/C)r and (Mb/C)e-

A plot of the degree of sensitivity of body 50

temperature to changes in ambient temper-

ature (ATb/ATa) as a function of body size

(Mb/C)e in some small birds.

The relation of body weight and the ratio 50

(Mb/C)r to the lower limit of thermoneutral-



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



Charles Gerald Yarbrough

June, 1970

Chairman: David W. Johnston
Major Department: Zoology

Data from the present study and the literature show

that the energetic of thermoregulation in small birds

are accurately described by the Newtonian model of heat

loss. The level of body temperature is closely correlated

with basal metabolic rate, thermal conductance, and body


Tropical and desert birds generally have lower relative

thermoregulatory quotients than expected (less than 1.0)

on the basis of weight alone, and cold-climate species have

high relative ratios. The relative ratio is a measure of

the impact of ecology and climate on thermoregulatory

capacity. Larger body weight can compensate for a low

relative ratio, within limits.

Bird data now available indicate that ecology,

particularly food habit, limits the distribution of birds

outside the tropics and deserts, and that thermoregulatory


parameters are adapted primarily to climate.

Studies of the energetic of Tb regulation in non-

desert tropical birds should give information as to the

impact of ecology on this process, in that the climatic

variable would be absent.


Thermal homeostasis and its energetic cost for

endothermic animals have been extensively documented over

the last 20 years. The Newtonian model of heat loss has

been used as a unifying method of looking at some of the

parameters of body temperature regulation. This relation-

ship is usually stated as:

dQL/dt = C (Tb Ta)

in which (dQL/dt)l is the rate of heat loss, Tb is body

temperature, Ta is the ambient temperature, and C is a

proportionality factor known as thermal conductance. In

thermoneutral or cooler ambient temperatures with no

radiant energy source, it is obvious that (dQL/dt) = rate

of heat gain = metabolic rate (M), if the animal is going

to hold Tb constant.

The zone of thermoneutrality for an endotherm is a

range of Ta over which M is constant and independent of

T If the animal is postabsorptive and quiescent, M is

equal to the basal metabolic rate (Mb) over this Ta range.

In this zone, heat balance is maintained by physically

changing the conductance (e.g., fluffing or compressing the

fur or feathers). Ambient temperatures below the thermo-

1 All symbols and expressions used in this paper are defined
in Appendix A.

neutral zone result in an increase in M, and the pro-

portionality of this increase to the (Tb Ta) difference

is a measure of C. Ta above thermoneutrality also result

in increased M values, mainly because of muscular activity

associated with panting and the van't Hoff effect on

chemical heat production.

The energetic of mammalian Tb regulation are better

understood than those of birds. This stems from a number

of reasons. Interest in avian thermoregulatory studies

developed rather slowly, major stimuli being the series of

papers by Scholander et al. (1950a, b, c) and studies of

hummingbird metabolism by Pearson (1950). Avian species

studied subsequently have not been as representative of

the diversity in size, ecology, and distribution within

the class as has been the case with mammals. In many

instances, avian thermoregulatory studies have been in-

complete because of the absence of precise measurements of

the Tb that is being regulated. Failure to report precise

Tb values for M measurements often will introduce bias

into calculations, and greatly reduce the further useful-

ness of the data.

The review of energetic and thermoregulation in birds

by King and Farner (1961) has been of considerable value in

giving direction and perspective to this area of investigation.

From the data then available, they derived an equation

relating b and body weight (W) in small birds (ca. 100

grams or less) and another equation for birds larger than 100

grams. A standard equation relating conductance and W

in birds is given by Lasiewski et at. (1967). Both of

these equations will be discussed later.

A few attempts have been made at analyzing thermo-

regulation in terms of environmental demands, especially

heat stress and evaporative cooling (e.g., Bartholomew

et aZ., 1962; Calder, 1964; Dawson and Fisher, 1969; Ligon,


Scholander et al. (1950c) have placed the emphasis on

insulation for thermoregulatory adaptation to low Ta. It

is now apparent (for example, Hart, 1957) that the basal

metabolic rate may also be adaptive. The adaptive nature

of body temperature is equivocal.

From Newton's heat loss relationship, one could

anticipate that in response to environmental stress

(thermal and/or nutritional) an animal could (1) behaviorally

or physiologically alter the (Tb Ta) difference, (2)

physically change its conductance, (3) chemically alter the

rate of metabolism, or (4) achieve some suitable combination

of these features. The behavior of these parameters in

relation to ecology and climate has been investigated in

some mammals, particularly by McNab (1966b, 1969, 1970;

McNab and Morrison, 1963).

The relationships among thermal and ecological factors

involved in thermoregulation have not been critically

studied in birds. Ecological and distributional correlations

have been made with differences in Mb and/or C in only a

few species (Wallgren, 1954; Lasiewski and Dawson,

1964; Johnson, 1968; Ligon, 1968). No attempt has been

made to synthesize into a rational whole the relation-

ships that may be of general application in birds. McNab

(1966a) has suggested that Tb in birds is determined by

the ratio of Mb : C, much the same as in mammals. He has

also indicated (1969) that in neotropical bats the (Mb/C)o

ratio, and thus thermoregulatory capacity, is ecologically

determined, especially by the reliability of the food


The present study compares the behavior of thermo-

regulatory parameters in response to thermoneutral or

cooler Ta for representatives of three avian families

having different food habits. It was suspected, in view

of the great mobility of temperate zone birds, that food

habit might be more influential on distribution than on

the thermoregulatory capacity. These studies on sparrows,

warblers, and flycatchers were undertaken to explore this

possibility. Representative data on thermoregulation from

the literature have been incorporated in the analysis of

physical and ecological correlates of general application

in birds.


All specimens used in this study were caught in mist

nets. Most of the flycatchers were 'obtained in the coastal

plain of North Carolina, the Harris' SparrowsI near

Lawrence, Kansas, and the Black-throated Trogon along the

Rio Negro near the junction with the Rio Branco in the

state of Amazonas, Brazil. All other birds were caught in

the vicinity of Gainesville, Florida, during 1968-69. The

time of year in which measurements were made for each species

is indicated in Appendix B.

The Harris' Sparrows were maintained in captivity,

but all other specimens were caught during the day (usually

afternoon) and were used for metabolic determinations the

same night. Small birds were fasted for at least five hours

and large birds six to seven hours before data collection

was begun. Only data obtained between 2100 and 0500 hours

were included in the study. Specimens were kept in complete

darkness during the study interval. All data from birds

suspected of being in Zugunruhe were excluded.

Metabolic rate was measured at three different ambient

temperatures: 100, 200, and 30.50 C, or as otherwise indica-

ted in a few cases. An open flow system was employed, and

Scientific names of all birds used appear in Appendix B.



air flow rates were sufficient to maintain a concentration

of at least 20 per cent oxygen in the metabolism chamber.

This chamber had a volume of one gallon, and was immersed

in a large, constant-temperature water bath. When a tem-

perature change was desired, the chamber was switched to

another water bath preset at the desired temperature. An

interval of about two hours was allowed for the bird to

adjust to the new temperature before data were used.

The partial pressure of oxygen in the air stream was

measured by a Beckman G-2 Paramagnetic Oxygen Analyzer

after carbon dioxide and water had been removed. Oxygen

concentrations were recorded on a Honeywell strip chart


When a satisfactory metabolic reading was attained,

the bird was quickly removed from the chamber and the pro-

ventricular body temperature was measured by a YSI tele-

thermometer. The bird could be removed and its Tb measured

in less than one minute. Thus, each metabolic value has

a corresponding Tb reading.


Data obtained in this study from 29 species of small

birds are shown in Figures 1 14 and are summarized for

each species in Table 1. These figures depict, species by

species, the effects of different ambient temperatures on

body temperature and metabolic rate (as measured by oxygen

consumption). Metabolic measurements made within the zone

of thermoneutrality are basal levels (Mb). At Ta below

the lower limit of this zone, oxygen consumption increases.

The slope of a line describing this M increase with

declining Ta is considered to be the thermal conductance (C)

of the species. General patterns of these responses are

indicated later in this section.

For all species studied, the data indicate that 30.50C

is well within the zone of thermoneutrality. This is support-

ed by the fact that most of the metabolic records obtained

at this Ta were quite smooth, whereas records taken at 100

or 200 C usually showed oscillations between maximal and

minimal values, presumably due to periodic bursts of shiver-

ing or other muscular activity. Thus, metabolic values

taken at 30.50C are probably basal rates, and represent the

lowest constant level maintained for 15 minutes. For

metabolic measurements below thermoneutrality, the repeatable

minima are used. This procedure minimizes both Mb and C.

Fig. 1. Responses of body temperature and oxygen
consumption to ambient temperature in Spizella
passerina and Zonotrichia albicoZZis. The lower
part of the graph shows the relationship between
oxygen consumption (M) and ambient temperature
(Ta). Horizontal lines indicate the mean basal
metabolic rate for each species. The absolute
value of the slope of each line in the lower part
of the graph represents the mean thermal conduct-
ance for each species. The upper portion of the
graph gives the body temperatures (Tb) corresponding
to the M values below. The slanted line in the
upper part is a reference which equates the temper-
ature axes.

Fig. 2. Responses of body temperature and oxygen con-
sumption to ambient temperature in PassercuZus
sandwichensis and Ammodramus savannarum. See the
legend of Fig. 1 for explanation.

" x o a
, .. .:o

0 20 30 40
I1a (c

& i "/i


2 F

* -- -p
10 2.


X .X 39




!,i,,an= .-; r-lt, ,, t -. / o..ura. -.. -,-..


la (OL

('C )

Fig. 3. Responses of body temperature and oxygen
consumption to ambient temperature in Melospiza
melodia and Melospiza georgiana. See the legend
of Fig. 1 for explanation.

Fig. 4. Responses of body temperature and oxygen
consumption to ambient temperature in Pooecetes
gramineus and Passerella iliaca. See the legend
of Fig. 1 for explanation.


b /

S--A- ,za mnelodca I

o -an-
... x x _
.. .

10 20 30 40
Ta 0 c)

1 42

x 114

Tb( c)

4 A ..... t: I- li iliaca
i ........ ....... .... .. I

O , ........ +

S ... .. .

10 20 30 43
Ta LC)
**: /[
c / 3

Fig. 5. Responses of body temperature and oxygen
consumption to ambient temperature in Zonotrichia
querula and Zonotrichia Zeucophrys. See the legend
of Fig. 1 for explanation.

Fig. 6. Responses of body temperature and oxygen
consumption to ambient temperature in Parula
americana and Vermivora pinus. See the legend
of Fig. 1 for explanation.

S: 40

L '39

S Zonotrichia gu'a
4 Z leuco~phrs

, a. r ... .. o .. .= .



S..... ....._ "...v.. ;. .,. ...
Si -- "..., | ,

0 20 30 40
Ta Vc )

Fig. 7. Responses of body temperature and oxygen
consumption to ambient temperature in Dendroica
pinus, Vermivora celata, and Mniotilta varia.
See the legend of Fig. 1 for explanation.

Fig. 8. Responses of body temperature and oxygen
consumption to ambient temperature in Dendroica
coronata and Protonotaria citrea. See the legend
of Fig. 1 for explanation.

TalC )


*-~r t nsz stt~r 2'~S rt-t, s.

I ~ i 0-~ 1Z L~t~~TiJ IH~Jf~IUI LT

X ~
Kc :2

* :

L-cDocrdroica c ron3ta [
.-. Protonotarla c'itr


*'*>-:- ---~ ;1. '--

10 20 30 40

Ta (c )

Ttb(* )


J 36


Tb )

Fig. 9. Responses of body temperature and oxygen
consumption to ambient temperature in Wilsonia
citrina and Dendroica dominica. See the legend
of Fig. 1 for explanation.

Fig. 10. Responses of body temperature and oxygen
consumption to ambient temperature in Seiurus
noveboracensis and Seiurus aurocapilZus. See
the legend of Fig. 1 for explanation.


T ( *

,*3 Tb(C

37 nia. citri.
6 I""" 'De.. ) ndroica do inica

Ix I/

DC ""

: b (c*
x x

f I p .. I

S.. ..

i: lb( c)

" ... ..... .. ...... ..
1 A |-- SP2urus n rO I

1f^ "l ""-*0; r ; |
i i' ''s-0 2 ~ -^ (' c.)

- .. - ,.- ... .. . . . . .. -.-.... . .
10 -.-.--. 40 v .

1 a (O, )

Fig. 11. Responses of body temperature and oxygen
consumption to ambient temperature in Dendroica
palmarum and Trogon rufus. See the legend of
Fig. 1 for explanation.

Fig. 12. Responses of body temperature and oxygen
consumption to ambient temperature in Sayornis
phoebe and Geothlypis trichas. See the legend
of Fig. 1 for explanation.

S o ,a 42

0 20 t

coDendroca p. lmarum

Ii .. .... ..!x Y.

IC 2') 30 40


~x 3x

6 e - Sayornic s phoeba._ru

X ...G-.,- i--- G tri.nhas fj

S. . ....... r.
o I ... .

1 u 20 30 .
Ta (c)
1 020301

0 I ^;;;:~,* ^'

I i^^ -
f *_. ^ ^
^ -- -- *^ ^* *- ^ - ^-
,,~l (c)6r,

Fig. 13. Responses of body temperature and oxygen
consumption to ambient temperature in Empidonax
virescens and Contopus virens. See the legend
of Fig. 1 for explanation.

Fig. 14. Responses of body temperature and oxygen
consumption to ambient temperature in Myiarchus
crinitus and Tyrannus tyrannus. See the legend
of Fig. 1 for explanation.

- K 13
x' a II

i 3
'[" "

Emoildoifx vir !.c.n3

.0 ,- Co T "". i vre ,-.
o .... ---.. -

10 20 30 40

rM ] *. *

STb *
Ta ('"C)

*X *
f 40

. ii 3-


Ta (c )

So M I 0 m o m In mn

0 0 0 0 0 0 0 0 0 0 .4 0 0 0

S 0 0 0 0 0 0 0 0 0 a

S. . . . . .
-> O W 0% 0 0 0 0 a O 0 0 o 0 0

r f 0 U/i %0 %0 ( 0 %0 v 0%

0N E E E E N EN n o n %o EN --

c a In co % rl I I co In on a oo or

S0 .n N. .-4 . 0 .


D 'r %. N N .I EN E

0 co r, %D '4 0 I r E N N cr 0D

F.I r- %0 In -4 -4 EN EN % a
0 .-4 .-4 .4 ..4 .4 ,-4 -4 -I EN EN .-4 .-4 .-4 .-4
rn cc co a% EN 0 0 O W ( EN .1 %% c

4 aN a oo o co co N aN 4 a cc N o r
F| . .4 .- . (4 E E E .4 .In -.

oWE N %o a- In 0E
c: n f W U r 0 0 W o U- r-A rn % o I
o t cu C4 .4 ^ .- o4 .4 -4 -4 r 0t EN E 4

SU 0 0 0 0 0 0 0 0 0 0 0 a 0 00


F-4 4 0

(y .3 (I o % ^ 0> ' 0o rL E- q5 W

C) o U .3

0. C .
0 U E 0 U(*

UI t. a oj (o 4 .

0u .. 0 1 0 0 0 h C) C E

C) I- a, U - t3 '4 F, F. ...

3. 0 .< a) 3 U 0 0 . U O U C) 0
0. Q. fl
tF -

N ooo W oo'

n I I I I I + I

Cm (4 0

4( ,( -1

..4 r- 4 rs

0 0 C 0

M (N fn
-4 ,-4 -4 -4

Wn\ f
I.- t-
r4 r* r-4

I (N

-4 -4

O Un fn w

4 -4 -4 -4

-4 -4 -4 -4

In -4 o f, f, (3 4 f, -
r w r w w co cO co

Oq *-

-4 -4 -

!' 6> W 1

Ln ,'D r- mi
r r 0 O a,









m a a


2 o *<


to '0 r
3 *

03 B . I c o

3 O '* 4 t. U U E O

o . O O 43 0

o 4-. .3 a . .4
.S r to 1L 13a
to E- 4. c c c 4 o E3
4. .3 .3 ..4 .3 .3 a M U '. 4. .
".3 o n. *4 U 4. a o JJ o'. *4 0 0 ?
4.. O "- o IV 0. c i t. t'
a . i 4. 14) u .3 o .
Ci Cl Q Cl U ^ 4.t U CO C. E-

r0 r0

o o







E< o
.0 0

O o
6- +


0 x

0 1

41 w

o o t

"0 U I 0 I

w > Q a o

0 0

I >

0 C U C
S.0 o

Eo -o


O0 0
0 )l X

o C o

U) ( 0 l

0 0 U I 0 I
3 c C) S i 'o 3
S*- 4 > o

Z*. 4. co W 3 l

S4, C Cn 4.4 C.

The mean Mb for ten species of sparrows is 92.6 per

cent of that expected from the equation of King and Farner

(1961) for small birds, and C is 75.0 per cent of the value

expected from the equation of Lasiewski et al. (1967) (Figs.

1 5, 15, 16, and Table 1). Of the families studied here,

the sparrows have the most northern wintering distribution,

and contrast with the tropical Black-throated Trogon, which

has a Mb only 77 per cent of that expected and a very high

C (161 per cent) (Figs. 11, 15, 16, and Table 1).

The mean Mb for all warblers is 89.2 per cent of that

expected and C is only 72.4 per cent of the expected value

(Figs. 6 12, 15, 16, and Table 1). Eight species winter-

ing as far north as the southeastern United States have

lower Mb and C values (85.8 and 66.5 per cent) than five

species wintering farther south (94.6 and 82.0 per cent).

These values indicate a considerable reduction in energy


The five species of flycatchers have a collective Mb

which is 100 per cent of that expected. C is 83.6 per cent

of that expected, and the Acadian and Wood Pewee have a lower

C (75.5 per cent) relative to that expected than the larger

flycatchers (89.0 per cent) (Figs. 12 14, 15, 16, and

Table 1).

Each metabolic datum is accompanied by a Tb value

(Figs. 1 14). The mean Tb for sparrows and warblers in

thermoneutrality was identical (40.3 C), but was less than

that of flycatchers (41.1 OC). Very small birds (<10 grams)

usually have Tb that are lower and more sensitive to cool

Ta than those of larger species.

Fig. 15. The relation of basal metabolic rate to
body weight in some small birds. The line is
a plot of the King-Farner (1961) equation for
small birds (log Mb = log 7.29 0.341 log W).
Each point is the mean value for a species.

Fig. 16. The relation of thermal conductance to
body weight in some small birds. The line is
a plot of the equation of Lasiewski et al. (1967)
(log C = log 0.848 0.508 log W). Each point
is the mean for a species.


0 x x

0 F IYatlrhers
X 'p r r roi'.-
A r rcon

I x iro




,'* arblerS
So Flycatchers
X Sparo, s

- Co >


X xy


r .



...... L.a ..- _, ..... L .'^ J..... ^ ... T-!. l..{ .... K.. .L._.J^J.^.'-- -J ^ t I. -.... I
0.9 1.0 1.1. 1.1.1 1.4 I 1.6 1.1

7 10 I5 ') 30 43 SO
W (g)


0.9 I.C '.I 1.2 II 1.4 1.5 1i. 1.1
LOG .*

I 10 15 ;0 30 4u 50
V. Ig)





Heat Loss and Heat Gain

Heat loss in animals is an extremely complex process.

The factor C conceals a multiplicity of problems concern-

ing heat loss by radiation, conduction, convection, and

evaporation of water. The effective body surface area may

be temporally varied, peripheral and appendicular blood

circulation is subject to alteration, and the temperature of

the body surface from which heat is lost may be different

than that measured in the core. Some of these problems

have been discussed in detail by Burton (1934), Scholander

et al. (1950a, b, c), Kleiber (1961), Bartholomew and Tucker

(1964), and McNab (1970).

It would be desirable to have a rigorous experimental

capability such that the physical parameters could be

measured which are necessary for using models such as

Fourier's law of heat flow. The ability to apportion properly

the responsibility for heat loss among radiation, convection,

and conduction as well as evaporation is also a worthy

aspiration. However, as indicated in the previous para-

graph, there are a number of reasons why this is impractical,

if not impossible, at present. It is important that we

begin to measure quantitatively the partitioning of heat

loss by birds. This partitioning is probably highly adaptive.

Knowledge of trends of performance by each avenue of heat

loss would allow us to understand the thermal and ecological

interrelationships of individual species. The recent

studies by Hamilton and Heppner (1967) and Heppner (1970)

on radiative energy exchange and its effect on the heat

budgets of dark and light birds are steps in this direction.

However, the details of the partitioning of heat loss

are overlaid by more general thermo-ecologic patterns. In

the final analysis, it is the total rate of heat loss that

must be balanced by heat production. It is exactly at this

point that the Newtonian model is useful. Its employment

simply ignores the various reasons for heat loss, and con-

siders the totality of heat lost as a single parameter.

This approach, although simplistic, allows examination

of the relationship of metabolic heat gain (M), and total

heat loss C (Tb Ta), to biological and distributional

characteristics of a species, such as ecology and climate. It

will be seen that the available data fit this simple approxi-

mation remarkably well. As long as the physical limitations

of this concept are understood, I believe that it can be of

considerable heuristic value in a biological sense.

The Newtonian model was originally used to describe

the cooling of a body. Since a decline in temperature does

not generally occur in homoiotherms, it is sometimes argued

that this law is not applicable to such systems. The argu-

ment becomes rhetorical when the cooling expression is re-

written as a heat loss statement using the specific heat (c)

and body weight (W):

dTb/dt = k (Tb Ta) (1)

dTb/dt = dQ/dt l /cW (2)

.*. dQ/dt 1/cW = k (Tb Ta) and (3)

dQ/dt = kcW (Tb Ta) or (4)

dQ/dt = C (Tb Ta) (5)

where C is a composite proportionality factor commonly

known as thermal conductance.


Of the species of birds used in this study, only a

few sparrows have been subjected to metabolic examination

by other investigators. Table 1 shows that sparrows

larger than 20 grams have Mb almost exactly as predicted

from W by the King-Farner equation, except for the Vesper,

which also has a lower Tb. King's (1964) data on the White-

crowned Sparrow agree with those obtained in this study.

Rising's (1968) values for Harris' Sparrow are much higher

than expected (158 per cent) and were obtained by weighing

CO2 produced. My data for the White-throated Sparrow are

similar to those of Hudson and Kimzey (1964). The values

given by Lasiewski and Dawson (1967) for White-throated, Song,

and Fox Sparrows are considerably higher than those from

this study. This may be due in part to differences in pro-

cedures of taking data from the strip charts. Figure 15

shows the position of my data in relation to the King-Farner

curve for small birds.

It will be noted from Table 1 that those sparrows

with %M lower than expected usually also have lower Tb than

the other species. Mb for most of the warblers are also


somewhat lower than expected, particularly in species winter-

ing in Florida or in very small species. This reduces

energy expenditure considerably and may be very important

for small insectivorous species wintering in cool climates.

The reduction in Mb is made possible in such species by a

comparatively better insulation. The two species of insect-

ivores that winter in the southeastern U. S. and still main-

tain relatively high Mb (Phoebe and Myrtle Warbler) are

known to depend heavily on berries for food during winter

(Bent, 1942, 1953; Yarbrough and Johnston, 1965).

The tropical and desert species in Table 2 have sig-

nificantly reduced Mb values in most cases. This may

function mainly for reducing energetic demands (small

hummingbirds) or for moderating the heat load and evaporative

water loss (desert owls and goatsuckers). The species

which appear to take exception to such strategies are either

very small (Black-rumped Waxbill) or have exceptionally

high C values, even in view of their small body size

(Paradise Widowbird and Zebra Finch).

Lasiewski and Dawson (1967) have statistically separated

standard metabolism curves of passerine and non-passerine

birds. No biological rationale is evident for this decision.

Indeed, there may be differences in Mb between passerines

and non-passerines; there probably are different curves for

penguins and hummingbirds, auks and vultures, etc. Non-

passerines have been inadequately sampled over the size

range from which most of the passerine data have been

rr r-

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obtained. Perhaps even more important than body size are

the climatic and ecological factors impinging on the birds

in question. It might be expected that birds from the

tropics or arid areas with great heat loads, soaring or

gliding species, flightless birds, would have reduced

metabolic heat production when compared to active foragers

contending with cool, cold or variable Ta in the absence of

a capacity for torpor. Many more data are needed for

passerine and non-passerine birds having similar distri-

butional histories, ecological characteristics, thermal

environments, and body sizes before it can be said that

physiological responses are determined by taxonomy at the

ordinal level.


As has been seen above, it is sometimes meaningless or

even misleading to view heat gain without evaluating heat

loss. I have already indicated that the procedure of data-

taking used here minimizes C. Data are available in the

literature for only two of the species studied by me.

King's (1964) C data on the White-crowned Sparrow are similar

to those reported here, but Rising's (1968) C values for

Harris' Sparrow are nearly double those in the current study.

The mean C for every species in the present study is

lower than that predicted by the equation of Lasiewski et al.

(1967), except for the Black-throated Trogon (Fig. 16).

This completely tropical species has a C on a par with the

Spotted Nightjar of Australian arid lands (Dawson and Fisher,


It seems to be generally true that birds from the

tropics or hot deserts have relatively high C values, and

species that encounter cool weather have low C. However,

there are some prominent exceptions to this generalization.

Very small desert or tropical species, such as humming-

birds may not be able to tolerate increases in C (also

compare the Black-rumped Waxbill with the larger Paradise

Widowbird and Zebra Finch). The smaller caprimulgids with

more temperate wintering distributions (Poor-will and

Pauraque) have relatively better insulation that the larger

Nighthawk and Spotted Nightjar that winter at lower


The small nocturnal owls have good insulation (Table

2). Of course, the Saw-whet is found in cold climates, and

the Whiskered and Elf Owls are active during the cool

desert nights. The only small owl in Table 2 with a rela-

tively poor insulation is the Pygmy, which is largely diurnal

in its activity and thus is exposed to the high Ta of the

southwestern U. S. and Mexico.

The importance of providing Tb measurements along with

M values is seen in the calculation of C. Some species

of birds probably do not change abruptly from physical to

chemical thermoregulation (Figs. 7, 9, 12, 13). Many species

also experience a reduction in Tb when Ta is below the thermo-

neutral range. These effects are additive, and under such

circumstances the slope of the regression line through the

data points does not represent C. Minimal C is the slope of

a line connecting the mean M value at a given Ta below

thermoneutrality with the corresponding mean Tb at M = 0.

The lower limit of thermoneutrality for purposes of calcula-

tion can be determined by translating this line to the right

on the x-axis to the point which is the Tb measured at

thermoneutral Ta.

The Physical Model

The relation of Mb to C for birds in the current study

is compared to a line representing predicted values (Fig.

17). It has been stated previously that this relationship

of heat gain (M) to the coefficient of heat loss (C) is a

better expression of the thermoregulatory capacity of an

animal than either parameter alone. Each of these factors

can probably be modified by selection and/or acclimatization,

but only with proper reference to the capacity of its com-

panion factor and body size. For example, the response to

an environmental alteration of C can be made only to the

extent that body size is not limiting and that the animal

can obtain an adequate food or water supply to operate at

the new M level. All this must be done within the frame-

work that exists for maintenance and reproductive success.

Therefore, the observed thermoregulatory quotient

(Mb/C)o is broadly composed of two components: (Mb/C)e, the

quotient expected of a "typical" bird of the specified W,

and (Mb/C),, an index of that variation from "typical"

that may be attributed to the adaptive impact of ecological

and climatic demands. The modified heat loss equation can

Fig. 17. The relation of the observed thermoregulatory
quotient (Mb/C)o to body weight in some small
birds. The line is a plot of the quotient expected
on the basis of weight from the equations of
King and Farner (1961) and Lasiewski et al. (1967),
log (Mb/C)e = log 8.59 + 0.167 log W.

Fig. 18. A plot showing the effect of body weight
on thermoregulation data taken from birds used
in this study and from the literature. Lines
have slopes equal to (Mb/C)e. Data for the
White-tailed Ptarmigan (Lagopus leucurus) are
from Johnson (1968). Symbols are as in Fig. 20.
See Tables 1 and 2 for data and references.

Ie t x Z "

SL2 2<

P ( r, t 6,(), (D

;* X o" o

10 -


I -
S7i IT 1.3 I.I I I I.6 I.I

lOG V.
I -, 2. + -
w ( P.)

I ,- I u -I

- /e

I0 - -

(,. .<),

be rearranged and partitioned so that:

Tb = (Mb/C)o + K and (6)

Tb = (Mb/C)r (Mb/C)e + K (7)

where K is the lower limit of thermoneutrality (McNab, 1969,


Since equation (7) has the general form y = ax + b,

the fit of available data to this model can be checked.

A biological check is also possible by rearranging equation

(7) so that:

Tb K = (14b/C)r (Mb/C)e (8)

where Tb measurements on the left side of the equation are

compared with metabolic measurements on the right. Graph-

ically represented, when (Tb K) is plotted against (Mb/C)e,

a line from the origin having a slope of (Mb/C)r should in-

tercept the corresponding data point (Fig. 19). In a more

dramatic way, when (Tb K) is plotted against (Mb/C)r

(Mb/C)e (equation 8), the data should fall along a straight

line having a slope of 1.0. In Figure 20, which is a plot

of this relationship, the closeness of fit is evident.

Determination of the Level of Tb

It is apparent that K is determined by Mb and C, both

by the weight factor (Mb/C)e and the relative ratio (Mb/C).r

Therefore, adaptation to climatic and ecological conditions,

along with W, sets the lower limit of the energetic quantity

(Tb K). This difference is the buffer between the bird's

regulated Tb and the necessity for increasing energy expendi-

ture. The magnitude of (Tb K) is a good indicator of the

Fig. 19. The relation of the extent of the thermal
buffer to the thermoregulatory quotient expected
from body size alone. Numbers at the left
terminus of each line are slopes. The number
accompanying each point is the (Mb/C)r value. The
mean (Mb/C)r value for encircled points is equal
to the slope of the line through the circle.
Symbols are as in Fig. 20. See Tables 1 and 2
for data and references.

35 r

30 [-




,1 i



I -
12 14 16 18
S rg 4a0.157

extent of cold stress to which a bird is subjected in its


The question as to whether Tb is a dependent or in-

dependent parameter is more complex. It is logical to

agree with Hammel's (1968) contention that an initial step

in the evolution of homoiothermy was necessarily the acquisi-

tion of an adequate insulation. Then, it becomes energetically

feasible to invest in a fairly high Mb. The eventual thermo-

regulatory quotient and size of the endothermic avian stock

likely resulted in a (Tb K) that was satisfactory for the

thermal environment in which these animals lived. This

original Tb would have been dependent on Mb, C, and W. As

the advantages of thermal homeostasis asserted themselves

(whatever they may be), it is reasonable to assume that

temperature-specific neural, enzymatic, and other systems

also evolved. These systems could be more efficient if Tb

were precisely regulated. So, there must have been a

parallel evolution of the energetic machinery making endo-

thermy possible and a regulating system that would maintain

Tb stability in the interest of physiological efficiency.

Ontogenetic evidence lends support to this speculation.

Nestlings of some altricial species of birds show a

chronological improvement in insulation, and at some rather

well-defined stage begin a rapid development of metabolic

heat production and thermoregulatory capacity (e.g., Dawson

and Evans, 1957, 1960; Yarbrough, 1970b). In cool environ-

ments, such nestlings first are able to regulate their Tb

at a lower level (e.g., 31 33 OC) than that of the adult

(about 40 C). The regulated Tb increases with age until

the adult level is achieved. So, it would seem that the

regulatory mechanism becomes functional at a stage when

(Mb/C)o and W reach an acceptable level, but the set point

of the thermostat (the reference temperature for the

regulating mechanism) is elevated as (Mb/C)o and W increase.

For the original data presented here, as well as for

the data from the literature, equation (6) predicts Tb

with an average error of only 0.3 OC. One might well have

expected experimental and mechanical error to exceed this

value. Thus, the level of Tb is very closely correlated

with the interaction of Mb, C, and W in birds, as it is in

mammals (McNab, 1966b, 1969). However, I feel that the

existing evidence does not warrant classification of Tb

as a completely dependent factor, except possibly in an

evolutionary sense. Of course, it is true that in thermo-

neutrality a given Tb depends on a particular balance of C

and Mb, but the Tb set point may influence the level of Mb.

The distinction lies between physical and physiological

dependence. In the absence of the proper interrelations

among Mb, C, and W, the thermostat is ineffectual. Under

usual circumstances the regulating mechanism acts as a fine-

tuning device within the limits prescribed by the thermal


It may be that the Tb set point is an integral part of

the evolutionary thermoregulatory complex, and, as a result,

is to some extent adaptive. (In this case, Mb, C, W, and Tb

might be more accurately termed interdependent.) Scholander

et al. (1950c) and most subsequent investigators have

concluded that Tb is non-adaptive, but there is now some

evidence that mammalian Tb levels may be capable of some

adaptive variation independent of C, and W (iMcNab,

1970). However, the avian class apparently shows much less

variation in Tb than is the case in mammals. This reduces

any adaptive effect of Tb, and increases the significance of

ecological and climatic impact on Mb and C.

Body Size Effects

The impact of W on thermal relations is considerable.

This is largely due to the change in surface : volume ratio

with weight. Surface area (A) is theoretically the most

satisfactory unit for heat loss calculations, except that it

is practically impossible to get an accurate estimate of A.

This is especially true in birds, where one would have to

decide what surface area must be considered. For example,

what relation does A of the exposed parts (legs, head, beak)

bear to the feathered surface? Alterations in peripheral

circulation, particularly to the extremities, and postural

changes complicate definition of the effective surface area.

Use of kW2/3 as a value of A does nothing to solve these

problems; it simply obscures them. Therefore, it is con-

sidered that, at present, W is the most undeceptive unit

for discussing body-size relations in birds.

The relationship of Mb to W in small birds (<100 grams)

has been described empirically by King and Farner (1961)

as following an equation equivalent to :

1b (cc02/g hr) 7.29 W-0.341 (9)

where W is in grams. Likewise, Lasiewski et al. (1967)

considered C to be related to W (grams) by the equation:

C (cc02/g hr OC) = 0.848 W-0-508 (10)

which is similar to the C equation of Herreid and Kessel

(1967). Thus, both Mb and C increase as W decreases.

However, Mb increases less rapidly with small size than

does C, and the thermoregulatory quotient increases as

W increases:

(Mb/C)e = 7.29 W-0.341/0.848 W-0.508 (11)

= 8.59 W0.167

where W is in grams. So, it can be seen that a species can

attain a given (Tb K) either by adaptively changing

(Mb/C)r or by possessing a suitable body size (Mb/C)e. The

effect of W on (Tb K) when (Mb/C)r is constant is shown

in Figure 18.

McNab (1969) suggested that there is a "critical"

weight in mammals below which Tb is dependent on W, but

above which Tb appears to be independent of W. Departure

of (Mb/C)r from 1.0 can compensate for W changes, and thus

alter this "critical" weight. A "critical" weight is also

indicated for birds (McNab, 1966a, 1970). So far as can be

determined from published accounts and the data presented

here, Tb in birds appears to be W-dependent throughout the

size range, with the highest Tb values occurring at about

20 grams. Above this size, Tb declines very gradually, and

below it, precipitously. Much of the data scatter is

probably due to the (Mb/C)r effect. Apparently the "critical"

weight is also variable. In warblers it occurs at 10-11

grams, in flycatchers at about 15 grams, and at 18-20 grams

in sparrows. The significance of these apparent differences

is not now obvious, although it may be similar to the case

in mammals (McNab, 1970) in which temperate species have

lower K values than tropical forms.

Precision of Th Regulation

The birds studied to date fall into two categories:

(1) those that regulate very well at moderate Ta at all times

until W (energy) loss results in weakening and death, and

(2) those that regulate very well at Ta of 10-30 OC but

can go into torpor in times of stress or inactivity (humming-

birds, goatsuckers, some swallows and swifts). Thus Tb in

birds would superficially appear to be less sensitive to

Ta changes than is the case in many mammals. It should be

noted, however, that most of the species of birds which one

would suspect as being poor thermoregulators are found in

the tropics. The physiology of tropical birds is essentially


Birds employed in the present study are adequate to good

thermoregulators. The change in Tb per unit change in Ta

(ATb/ATa) is a measure of the sensitivity of Tb to Ta.

Figure 21 shows that there is little change in Tb over Ta from

300 10C. Although there is a greater change in the smaller

species of each family or those having a low (Mb/C)r, the

poorest thermoregulator is precise when compared with values

for some mammals (McNab, 1966b, 1969). Values of ATb/aTa >

0.10 are found only in those species with both very small

Fig. 20. A plot of the extent to which body temperature
is buffered, as a function of the combined ratios
(Mb/C)r and (Mb/C)e. The line is expected from
Newton's law of heat loss. See Tables 1 and 2
for data and references.

28 -

f Warblers
2 Flycatchers
4- x Sparrows
Al ummingbir
I Caprimulgid
r'Srnall Fincd
20 l Vidua
S El Crossbills
in Jays
i- Smnall OI wlI
1 I Evening Gr

12 L









.8 12 16

(C Ir

20 24 28 34 36

(M "l

Fig. 21. A plot of the degree of sensitivity of body
temperature to changes in ambient temperature
(ATb/ATa) as a function of body size (14b/C)e in
some small birds.

Fig. 22. The relation of body weight and the ratio
(Mb/C)r to the lower limit of thermoneutrality.
Arrows indicate the climatic zone which is the
wintering limit for each general food habit type.
See the text for further explanation, and Tables
1 and 2 for data and references. Symbols are
as in Fig. 20.

Ta 30.5 L_


1.1 1.16
*1.10 *1.16


1.32 1.29

o .14

X.1 1.1


o Flycatchers
X Sparrows



1.00 X.26

0.10 -

-- .1.03

..l. /JlLLL.LJ^ _jJlILLI__ 1 1 1 1 1 I1 I 1 1 1 1 1 1 1 1 1


MC e

S8.59 W 16

Food habt non- lhmilng )

Wintering large. aerial insect ivores
-tlh caparcty for lorpor
Nectar feeders _o -
Y.intering foragilg inscti vores 0 0

facultativi e.

Wintering gianivores
and Carnivores


------ --------------~---------I-- -uEl~a;rrro~-~slrua~la~wrru-wran


- -


body size and low (Mb/C)r, the Parula Warbler and the

Acadian Flycatcher.

-The amount of energy reserves (fat) is an important

factor in the precision of regulation. Weight deviations

greater than five per cent from the species mean are

positively correlated with Tb deviations greater than

0.5 OC from the species mean. Such an analysis will account

for 83 per cent of the variation in warblers, 77 per cent in

flycatchers, 61 per cent in Zonotrichia sparrows, and 100

per cent in sparrows smaller than 20 grams.

Adults of small species with altricial young probably

live near the limits of their energy reserves during the

breeding season, particularly during the nestling period.

This is an especially acute problem for insectivorous

species, if that season happens to have frequent inclement

weather. The frequency of thunderstorms during the nestling

period of 1969 was abnormally high in eastern North Carolina.

Results of several metabolic tests of the small flycatchers

(particularly the Acadian) had to be discarded because the

birds were in such poor nutritional condition. If Acadians

were captured before noon prior to nocturnal testing, some-

times they would weaken and die by the following morning.

Even in a species that forages and flycatches, such as the

Blackpoll Warbler (Dendroica striata), body lipid content

may be almost completely exhausted during parts of the

breeding season (Yarbrough, 1970a).

Ecological and Distributional Relationships

Some of the ways in which ecology and climate may affect

the behavior of various thermoregulatory parameters have

been discussed above. It is my intent in this section to

summarize some of the general relationships that exist

between the physical model and patterns of avian dis-


Consideration of all the factors in the heat loss

model accurately accounts for non-experimental variation

in the data (Fig. 20). The various species spread out

along the theoretical line, with very small birds or desert

and tropical species nearest the origin, and birds that are

larger or adapted to cold Ta farthest from the origin.

A more meaningful representation of this information

is given in Figure 22. The lowest thermoneutral Ta for a

species is connected with the origin. The data point is

placed where this line intersects a line between the

weight and (Mb/C)r characteristics for the species under

consideration. These thermoregulatory parameters can then

be related to climatic or latitudinal distribution. The

climatic zone which is the winter thermal limit for birds

having certain food habits is also indicated. The picture

could be more precise if we understood the impact of

ecological factors other than food habit on the distri-

bution of birds.

Lines A, B, and C are empirical thermo-ecologic

boundaries for small birds up to about 100 grams in weight.

Movement from one zone to another is possible by changing W

or (Mb/C)r. For example, the birds in the tropical-desert

zone are not generally cold-stressed in nature and have

become adapted for coping with. heat stress and water scarcity.

This may involve being very small (small finches and Paradise

Widowbird) or having a low thermal index (Tb K), such as

that of the Black-throated Trogon and some caprimulgids.

The capacity for torpor serves as a buffer to reduce energy

outlay in times of food deprivation and occasional cool


Birds wintering in the cold-temperate to subarctic

zone must have a high (Mb/C)r and cannot be smaller than

about ten grams. The thermoregulation of northern chickadees

(Parus hudsonicus and P. cinctus) should be examined from

this standpoint. Food habit is usually limiting in this

zone, since only herbivores and carnivores (including scaven-

gers, such as gulls and ravens) are found here. Steen

(1958) has found that some small, cold-climate species

can allow Tb to drop as low as 30 "C at night as an energy

conservation measure. He also suggests that under natural

conditions these birds avoid severe cold stress by means of

their roosting habits.

The majority of species used in this study fall into the

temperate zone, having K values between 220 and 250 C. It

is of interest to note that all the flycatchers studied,

except the Acadian, are thermally capable of wintering in

the temperate zone. This is an obvious case of avian dis-

tribution being limited by food habit (and possibly evolu-

tionary history). The warblers that winter as far north as

the southeastern U. S. fall into zone C (temperate), where-

as those that winter from the Caribbean area southward

fall into the subtropical zone on the graph. The winter

range of the Pauraque does not extend as far north as that

of the Poor-will. Yet, on a thermal basis the Pauraque is

much better equipped to regulate at low Ta. It would be

germane to know if the Pauraque can or does enter torpor as

readily as the Poor-will. The lower Mb of the Poor-will

may indicate a greater external heat load than is true for

the Pauraque.

The three small owls of the southwestern U. S. and

Mexico that have been studied are distributed as one would

predict from Figure 22. The Saw-whet has thermoregulatory

characteristics that coincide with its actual cold-temperate

distribution. The Giant Hummingbird has the thermal capa-

bility for a subtropical distribution, instead of tropical

as is the case with smaller hummingbirds. Its actual dis-

tribution extends from the montane tropics southward into

Peru and Chile. Range extension of some hummingbirds beyond

the thermal limits suggested by their thermoregulatory

parameters is probably due to their capacity for torpor.

The present state of knowledge concerning Tb regulation

in birds does not allow conclusions to the effect that food

habits determine thermoregulatory capacity, as they apparent-

ly do in bats (McNab, 1969). Food habit mainly affects the

distribution of most birds studied to date. Species with

seasonal food problems usually migrate; thus, thermoregulatory

adaptation is unnecessary. There may be correlations of

food habit with Mb and C in some species, but other factors

such -as heat and water stress would complicate any analysis.

In birds, it appears that thermoregulatory parameters

are adapted primarily to climate, and are only indirectly

or secondarily related to food habit. Therefore, large

climatic differences must not be introduced into data that

are used to determine the impact of ecology on thermoregu-

lation. Such data have not yet been gathered. Consequently,

the need is obvious for extensive studies on tropical birds

of all sizes, ecological characteristics, and taxonomic



The energetic of Tb regulation in small birds are

accurately described by the Newtonian model of heat loss.

A thermoregulatory quotient (Mb/C)o which considers both

heat production and the coefficient of heat loss is a more

satisfactory indication of thermoregulatory capacity than

either parameter alone.

Climatic and ecological adaptations of these parameters

are indicated by (Mb/C)r, which compares (Mb/C)o to the

value (1.00) expected from W alone (Mb/C)e. Tropical and

desert species generally have low (Mb/C)r, and birds that

must tolerate cold climates have high values. Birds living

in cool climates may partially compensate, within limits,

for low (Mb/C)r by having an increased body size (Mb/C)e, the

capacity for torpor, or both.

Mb may be lower than expected in desert species

mainly to reduce the heat load and evaporative water loss.

Tropical, non-desert species may also have a lower Mb than

expected for other birds in order to reduce the heat load,

to conserve energy (if the food supply is unreliable), or

simply because they are not faced with the need for a

higher rate of heat production. Some cool-climate birds

may also have a reduced Mb to conserve energy. This is

particularly true if the birds are very small and dependent


on a variable food source (such as insects). In this case,

C is also reduced. It is suggested that Mb differences

between any two avian groups are actually correlated with

ecological and climatic differences in the birds sampled,

not taxonomy per se.

C is generally high in tropical and desert forms, and

low in cold-climate species. However, the effect of

environmental stress cn b may militate against this

strategy in some cases. Likewise, 4b cannot be radically

altered if the environment precludes compensatory changes

in C. Mb and C must evolve as complementary thermal


The level of Tb is very closely correlated with Mb, C,

and W. Non-torpid birds are very precise thermoregulators,

as compared to most mammals. A combination of very small

body size and a low (Mb/C)r may reduce the precision of Tb

regulation, as may a drastic loss of W during extended food


Presently available information indicates that the dis-

tribution of avian species outside the tropics and deserts

is limited by ecology, particularly food habits. Thermo-

regulatory parameters are primarily adapted to climate. Thus,

in order to determine the extent to which food habit can

affect thermoregulation, significant climatic differences

must be circumvented. The ecological impact on Tb regulation

in birds must remain poorly understood pending investigation

of tropical faunas.



Symbols and Expressions Employed in This Paper

Symbol Description Units



Body weight

Metabolic rate

Mb Basal metabolic rate

Mb% Mb% expected from W by the King-
Farner (1961) equation:

Mb (ccO2/g hr) = 7.29 W-0.341

C Thermal conductance

C% C% of that expected from W by the

equation of Lasiewski et at. (1967):

C (cc02/g hr OC) = 0.848 W-0.508

(Mb/C)o Observed thermoregulatory quotient

(Mb/C)e Thermoregulatory quotient expected

from W by the King-Farner and

Lasiewski et aZ. equations (see

above): (Mb/C)e = 8.59 W.167

(Mb/C)r Relative thermoregulatory quotient

(Ib/C) o/ (Mb/C) e

Tb Body temperature

Ta Ambient temperature


cc02/g hr

cc02/g hr

Per cent

cc02/g hr AC

Per cent








Symbol Description Units

Change in body temperature per unit

change in ambient temperature

Lower limit of thermoneutral zone

Extent of thermal buffering

Heat loss












ccO2/g or




cc02/g C or

cal/g 'C


Cooling constant

Surface area

Specific heat


Scientific and Common Names for Species
Discussed in This Study

Species for Which Original Data Are Presented


Spizella passerina

Ammodramus savannarum

Melospiza melodia

M. georgiana

Passerculus sandwichensis

Pooecetes gramineus

Zonotrichia albicoZZis

Z. Zeucophrys

Z. querula

Passerella i-iaca

Chipping (W)1

Grasshopper (W)

Song (W)

Swamp (W)

Savannah (W)

Vesper (W)

White-throated (W)

White-crowned (W)

Harris' (W)

Fox (W)


Empidonax virescens

Contopus virens

Sayornis phoebe

Myiarchus crinitus

Tyrannus tyrannus

Acadian (S)

Wood Pewee (S)

Phoebe (W)

Crested (S)

E. Kingbird (S)

1The symbols indicate the time of year when species were
captured and studied: (W) winter, (S) summer, (M) migrant,
not in Zugunruhe.



Wood Warblers

Parula americana

Vermivora pinus

V. celata

Mniotilta varia

Dendroica dominica

D. palmarum

D. coronata

D. pinus

Geothlypis trichas

Wilsonia citrina

Protonotaria citrea

Seiurus noveboracensis

S. aurocapillus

Parula (S)

Blue-winged (M)

Orange-crowned (W)

Black-and-white (W)

Yellow-throated (S)

Palm (W)

Myrtle (W)

Pine (W)

Yellowthroat (W)

Hooded (S)

Prothonotary (S)

N. Waterthrush (M)

Ovenbird (M)


Trogon rufus

Species for Which Data Were Obtained

Otus trichopsis

Glaucidium gnoma

Micrathene whitneyi

AegoZius acadicus

Phalaenoptilus nuttallii

Nyctidromus albicollis

Chordeiles minor

Eurostopodus guttatus


from the Literature

Whiskered Owl

Pygmy Owl

Elf Owl

Saw-whet Owl



Common Nighthawk

Spotted Nightjar



Eugenes fulgens

Lampornis clemenciae

Patagona gigas

Vidua paradisea

Perisoreus canadensis

Cyanocitta cristata

Hesperiphona vespertina

Loxia curvirostra

L. leucoptera

Taeniopygia castanotis

Estrilda troglodytes

Rivoli's Hummingbird

Blue-throated Hummingbird

Giant Hummingbird

Paradise Widowbird

Gray Jay

N. Blue Jay

Evening Grosbeak

Red Crossbill

White-winged Crossbill

Zebra Finch

Black-rumped Waxbill


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Charles Gerald Yarbrough was born October 13, 1939,

at Lumberton, North Carolina. He was graduated from

Bladenboro High School (N. C.) in May, 1957. In June,

1961, he received the degree of Bachelor of Science with

a major in Biology from Wake Forest University. From 1961

until 1963 he was a teaching assistant and graduate fellow

at Wake Forest, where he received the Master of Arts degree

in Biology in June, 1963. He attended the University of

Michigan as a teaching fellow during the academic year

1963-1964. During the summer of 1964 he did ecological

research in the Canadian subarctic on a Chapman research

grant from the American Museum of Natural History. He

was employed as Instructor in Biology at Campbell College

for two years, 1964-65 and 1966-67. During 1965-66 he

worked as an Interim Instructor in Zoology at the Univer-

sity of Florida. From September, 1967, until the present

time he has pursued his work toward the degree of Doctor

of Philosophy while in tenure of a National Science Founda-

tion Traineeship at the University of Florida. In 1968, he

worked as an avian ecologist at an Atomic Energy Commission

project on Amchitka Island, Alaska.

Charles Gerald Yarbrough is married to the former Hazel

Ruth Hill, and is the father of two sons. He is a member of


Sigma Xi, Phi Sigma, the American Ornithologists' Union,

the Cooper Ornithological Society, and the Wilson Ornitho-

logical Society.

This dissertation was prepared under the direction

of the chairman of the candidate's supervisory committee

and has been approved by all members of that committee.

It was submitted to the Dean of the College of Arts and

Sciences and to the Graduate Council, and was approved as

partial fulfillment of the requirements for the degree of

Doctor of Philosophy.

June, 1970

Dean, College of Arts and\S iences

Dean, Graduate School

Supervisory Committee:

Chairman I

/M(}i ^ ~d i^. 5.C


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