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CONSEQUENCES OF NESTING DATE ON NESTING SUCCESS
AND JUVENILE SURVIVAL IN WHITE IBIS
JOHN DAVID SEMONES
A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE
UNIVERSITY OF FLORIDA
John David Semones
This project succeeded in part due to the support and physical aid of numerous
individuals and agencies. First, I would like to thank all the field technicians, volunteers,
and birdwatchers across the state of Florida and those who wrote to me with roost
information from Louisiana. These individuals provided invaluable aid in data
collection, monitoring of roost locations, and information on lost birds. My advisor,
Peter Frederick, and members of his research group, Becky Hylton and Julie Heath,
provided continuous input on the project's design and implementation, as well as support
in the field. Peter, in particular, kept me pointed in the right direction and taught me the
correct time and place for using the word "indubitably." His knowledge and enthusiasm
about wading birds and the Everglades are contagious and I now know more about engine
mechanics than most people rightfully should know. Throughout this process my
committee members, Ken Meyer and Madan Oli, provided feedback on my ideas and
helped make this a well-rounded project. Ken's advice on telemetry flights and radio-
tracking methods was particularly instrumental.
I am grateful to A.R.M. Loxahatchee National Wildlife Refuge, Everglades
National Park, and the Palm Beach County Waste Facility for permission to conduct this
research with unfettered access to these locations. Marian Bailey stands apart as a key
figure in facilitating a smooth flow of information, support, and general positive attitudes
while working in Loxahatchee.
This project required the use of loud and cranky airboats and small fixed-wing
aircraft on a near-daily basis. I thank Jamie Cloninger for keeping me afloat, Peter for
walking me through in-field boat repairs, and all my pilots for always returning me alive
to the ground. Next time I will take the bus.
My family and friends provided endless amounts of encouragement and an open
ear when necessary, I thank them for that. Finally, I thank my wife Traci, for her support,
kind words, and patience in putting up with my absence for such long periods of time
during this project are admirable. She makes it all worthwhile.
TABLE OF CONTENTS
A C K N O W L E D G M E N T S ................................................................................................. iii
L IST O F TA B L E S ......... ............ .......... .......................... ..... ... .............. .. vii
LIST OF FIGURES .................. ....................... ................. viii
ABSTRACT .............. .................. .......... .............. ix
1 IN TR OD U CTION ............................................... .. ......................... ..
M echanism s for a Seasonal Decline in Nest Success...................................................1
Differences in Temperate and Tropical Bird Reproductive Parameters.....................6
Subtropical South Florida and the Everglades................................. ............... 10
2 M E T H O D S .......................................................................................................14
Study A rea ...................................................................................................... ....... 14
Reproductive Success .................. ................................... .. ...... .. .......... 15
Defining Early and Late Parts of the Season...........................................................18
Juvenile Survival ................................................................. ........... 19
Transmitter Harness Design and Attachment..........................................................20
Tracking Radio-Tagged Birds ............................................................................. 22
Statistical A analyses ........................................................ ...... .. .... ...... .. 23
3 R E S U L T S .......................................................................... 2 5
Comparison of Reproductive Parameters In Early and Late Nests ............................25
M ayfield N est Success Estim ates ........................................ .......................... 28
N est Abandonm ent and Predation ........................................ ......................... 30
Survival of Young Post-Fledging ................................ ....................31
4 DISCUSSION ................ ......... ...... ...... ...............36
R productive P aram eters .................................... ............ ................ ......................38
M ayfield N est Success Estim ates ........................................ .......................... 38
N est Abandonm ent and Predation ........................................ ......................... 41
Post-Fledging Survival of Young ............... ....... ............ .... ..... .......... 44
5 CONCLUSIONS AND FUTURE DIRECTIONS ............................................. 46
A LOCATIONS OF COLONIES WITH MARKED NESTS IN THIS STUDY ..........58
B RADIO-TRACKING RESIGHT DATA FOR ALL MARKED BIRDS IN 2001
A N D 2 0 0 2 ........................................................................... 5 9
L IT E R A T U R E C IT E D ............................................................................ ....................62
B IO G R A PH IC A L SK E TCH ..................................................................... ..................70
LIST OF TABLES
1. Numbers of White Ibis nests marked by nest period and colony in 2001 and 2002.....26
2. Mean clutch size and hatchability (the percent of available eggs at the time of hatching
that actually hatched) comparisons from marked White Ibis nests..........................26
3. Comparison of mean White Ibis clutch sizes and Mayfield nesting success estimates in
this study with those from studies in other years and locations............................. 27
4. Comparison of White Ibis hatchability from marked nests in 2001 and 2002 with
marked nests from W CA 3 in previous years. ................................. ............... 28
5. Mayfield estimates of nest success rates for White Ibis nests monitored in 2001 and
2 0 0 2 ........................................................................... 2 9
6. Causes of nest failure in early and late-initiated nests monitored in 2001 and 2002....31
7. Distribution of radio-marked juveniles by nest period and mean age (in days) at
independence for all marked birds that survived to independence ........................31
8. Maximum reception distances (km) of transmitter signals from both ground and aerial
testing .............................................................................. 34
LIST OF FIGURES
1. Location of monitored White Ibis colonies in 2001 and 2002...................................48
2. Nest initiation ranges by nesting period for marked White Ibis nests in 1986, 1987,
2001 and 2002. .........................................................................49
3. Four year comparison of Mayfield nest success estimates of White Ibises nesting early
and late in the season .................. ............................. .. ..... .. ........ .... 50
4. Monthly rainfall, breeding chronology (proportion of marked nests initiated each
month), and nest failures (proportion of nest attempts that failed) of monitored
W hite Ibis nests in the interior Everglades.............. .............................................51
5. Multi-year comparison of Mayfield nesting success estimates for White Ibises in the
interior Everglades. .............................. ....... ...... ........ .... ...... ...... 52
6. Percent nest abandonment of White Ibis nest attempts (marked nests) in 2001...........53
7. Percent of White Ibis nest initiations (marked nests) that were abandoned in 2002....54
8. Comparison of overall nest abandonment rates at marked White Ibis nests for early
and late-nesting pairs in both study years (all colonies inclusive).........................55
9. Kaplan-Meier survival estimates for marked juvenile ibises in 2001.........................56
10. Kaplan-Meier survival estimates for marked juvenile ibises in 2002.......................57
Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science
CONSEQUENCES OF NESTING DATE ON NESTING SUCCESS
AND JUVENILE SURVIVAL IN WHITE IBIS
John David Semones
Chairman: Peter C. Frederick
Major Department: Wildlife Ecology and Conservation
Differences in temperate and tropical and/or subtropical avian reproductive
parameters may exist, making it difficult to apply temperate models of life history
strategies to tropical and subtropical species. It is unclear whether avian species breeding
in subtropical or tropical regions suffer the same seasonal decline in nest success
commonly associated with temperate birds. I designed a study to compare some of the
reproductive parameters (timing of breeding, nesting success, and juvenile survival) of
White Ibises (Eudocimus albus), a primarily subtropical and tropical-breeding bird,
nesting early and late in the breeding season. I monitored the fates of marked nests and
radio-marked juvenile ibises from the Florida Everglades in 2001 and 2002, two years of
contrasting hydrological conditions in the region.
When comparing years using the Mayfield method, White Ibis nesting success
varied inconsistently during the breeding season. In 2001, a very poor breeding year
overall (5.4% nest success), early-nesters had greater nest success than late-nesters (4.6%
and 1.7%, respectively). However, I observed a reversal of this pattern in 2002, a more
typical nesting year in the Everglades (44.5% overall nest success), as late-nesters had
greater nest success (44.9%) compared to early-nesters (33.4%). Early-hatched juvenile
ibises in both study years had greater survival compared to late-hatched ibises, at least
until 90 days post-independence from the colony.
The evidence from this study indicates that White Ibis nesting success in the
Everglades seems to vary inconsistently as the breeding season progresses. I suggest that
this inconsistent pattern of White Ibis nesting success was due to the unpredictable timing
of wetland ecological conditions and variability in the extent and magnitude of these
conditions in general, regardless of latitude, as opposed to a greater variation and
unpredictability in subtropical compared to temperate environmental conditions. In
either explanation, environmental inconsistencies between breeding seasons could make
it difficult for adult White Ibises to select the optimal time to initiate nesting attempts,
therefore potentially compromising nest success and offspring survival.
Many species of temperate breeding birds show a decline in nesting success and
offspring survival as a breeding season progresses, due to deteriorating environmental
conditions (Lack 1966, Perrins 1970, Verhulst and Tinbergen 1991, Norris 1993, Lepage
et al. 1999) and/or degrading food sources (Frederick and Collopy 1989a, Ogden 1994,
Martin 1996), poor experience in late-nesting birds (Verhulst and Tinbergen 1991,
Brinkhof et al. 1993), or some combination of these factors (Perrins 1966, Martin 1987,
1996, Price et al. 1988, Tinbergen and Daan 1990, Daan et al. 1990, Norris 1993). Most
research regarding breeding chronology and offspring survival in birds has been
conducted in temperate regions. It is unclear whether avian species breeding in
subtropical or tropical regions also suffer the same seasonal decline in nesting success
commonly associated with temperate birds. The models that have been proposed to
explain the phenomenon in temperate bird species may not be pertinent under the
different ecological conditions experienced by subtropical and tropical species, and the
mechanisms that influence life history evolution in temperate birds may not be present
and/or important in the tropics and subtropics (Ricklefs and Bloom 1977).
Mechanisms for a Seasonal Decline in Nest Success
Two mechanisms are more commonly suggested to explain why early-nesting
temperate birds are generally more successful than late-nesters. First, early and late-
nesters may experience differences in environmental conditions (Verhulst and Tinbergen
1991, Ogden 1994, Brinkhof 1997, Morrison 1999), possibly influencing reproductive
decision-making and life history optimization (Daan et al. 1990, Tinbergen and Daan
1990). These environmental constraints could include a decrease in food resources over
the breeding season (Sydeman et al. 1991, Ogden 1994), changes in predation pressure
(Lloyd et al. 2001, Gotmark 2002, Jobin and Picman 2002), the onset of winter (high
latitude sites) or the wet season (Perrins 1966, Frederick and Collopy 1989a, Young
1994a, Ogden 1994, Morrison 1999), and late-winter storms (Perrins 1966).
Late-hatched young may miss the period of most abundant food resources (Perrins
1970, Martin 1987), potentially retarding development. Sydeman et al. (1991) observed a
decline in fledging success over the breeding season in Western Gulls (Larus
occidentalis) nesting on Southeast Farallon Island. They correlated this decline in
fledging success with a seasonal decline in the abundance of rockfish (Sebastesjordani),
the primary food of chicks, though parental experience and individual quality may, in
part, have contributed to the observed seasonal variation in fledging success (Sydeman et
In a clutch switching experiment delaying or advancing the hatch date of Blue Tits
(Parus caeruleus), Norris (1993) found a decline in juvenile survival with hatching date.
Perrins (1970) suggested that if the period of greatest food abundance for tits is
sufficiently short, late-nesting tits might hatch chicks during sub-optimal periods of less
abundant resources. Hatching outside this window of peak food abundance may have
contributed to the observed decline in late-hatched juvenile success observed in Blue
Tits. Yet, differences in the genetic composition of chicks and/or parents, and the quality
of parental care also may have influenced the observed seasonal decline in Blue Tit
juvenile success (Norris 1993).
In many birds, juvenile size at fledging is important in determining post-fledging
survival (Perrins 1965). Lepage et al. (1999) found that the growth rates of Greater Snow
Goose (Anser caerulescens atlanticus) goslings near fledging were slower later in the
season compared to earlier in the season. This seasonal disparity in growth rates resulted
in the larger size and mass of early versus late-hatched goslings near fledging. They
attributed the differences in growth rates to environmental factors, most likely a seasonal
decline in food supply and colder late-season temperatures. Since the growth of goslings
hatched on the same date did not vary between control and manipulated clutches, it was
not likely that the quality of parental care contributed to the cause of the observed
seasonal variation in gosling growth rates. However, differences in parental care or
genetic differences of both chicks and parents may have influenced gosling growth rates
and, subsequently, survival. Although they did not find any differences in pre-fledging
survival during their study, there was an overall trend of a seasonal decline in pre-
fledging survival for this population (Lepage et al. 1999).
A second mechanism to explain late-season declines in nesting success and chick
quality (chick quality includes aspects of genetic composition and overall chick health)
involves a declining quality of parental care and parental quality (same as for chicks)
during the breeding season (Perrins 1970, Price et al. 1988, Brinkhof et al. 1997). An
association between early breeding and high nutritional state in adults has been
established for some birds, and females in good nutritional condition typically have
higher reproductive success (Lack 1968, Perrins 1970). In many species, older parents
tend to nest earlier and achieve higher reproductive success than younger parents (Finney
and Cooke 1978, Dow and Fredga 1984, Nisbet et al. 2002), although this observation
clearly does not separate age and nutritional state, since environmental factors could still
generate or contribute to the phenomenon.
In the colonial nesting Common Guillemot (Uria aalge), the onset of nesting can
be highly synchronous with other colony conspecifics, regardless of the date (Hatchwell
1991). This strategy may be advantageous because synchronous nesting could swamp
local predators, thereby reducing individual predation rates. Common Guillemots nesting
on Skomer Island, Wales, displayed a seasonal decline in nest success, partially explained
by late breeders' use of poor nest sites, increased predation of later and less synchronous
nesters, and partially by the poorer ability of late breeders to provision young, even
though there was no seasonal difference in food availability (Hatchwell 1991).
In a study of post-fledging survival, early-fledged European Coots (Fulica atra)
from cross-fostered clutches had increased first-year survival compared to late-fledged
coots (Brinkhof et al. 1997). Individual pairs that differed by 10 days in laying had either
the hatch date of their young advanced (late laying parents) or delayed (early laying
parents) by switching clutches with a different lay-period during incubation. Survival of
juveniles laid later in the season but cared for by early-laying parents increased to similar
levels as control (non clutch-switching), early-laying parents. Only a slight difference
was evident between the survival of juveniles cared for by advanced pairs and those of
controls. The experimental results supported the idea that differences in first-year post-
fledging coot survival were due to differences in the quality of parental care (parental
hypothesis). Yet, it is interesting to note that previous cross-fostering experiments with
European Coots (Brinkhof et al. 1993) found that a seasonal decline in coot nesting
success resulted from a combination of differences in the quality parental care of early
and late-nesters (parental hypothesis) and from potential effects due to the seasonal hatch
date (date hypothesis).
Separating the mechanisms of seasonal variation in nest success and post-fledging
survival, even in temperate regions, has proved difficult. While the date and parental
hypotheses have been tested with avian species breeding in temperate zones, there is a
lack of information regarding the general life histories and any potential within-season
differences in nest success and juvenile survival of tropical and subtropical breeding
birds. Very few tropical bird studies have differentiated between early and late-nesting
periods within a season (Morrison 1999, Ramos 2001, Olmos and Silva e Silva 2001),
and comparisons between temperate and tropical species generally report only the overall
trends in nest success and chick survival (Martin 1995, 1996).
Differences in temperate and tropical/subtropical avian reproductive parameters
may exist, making it difficult to apply temperate models of life history strategies to
tropical and subtropical species. In temperate regions, much of the seasonal patterns of
success are dependent upon seasonal differences in resource abundance (see next
section). The timing of nesting events, the window of breeding opportunity available for
successful nesting, and the magnitude and duration of changes in ecological conditions
may be as important as factors in nesting success and juvenile survival for
tropical/subtropical nesters as they are for temperate nesters.
However, the tropics may have a greater variation in the windows) of nesting
opportunity and less consistent timing of seasonal resources than temperate ecosystems.
How and when such changes in ecological conditions (e.g. seasonally fluctuating food
resources) occur during a breeding season could affect associated seasonal patterns of
nest success and offspring survival in the tropics. Quality of parental care in tropical
birds also may have a significant affect on within-season nesting success. If this is true,
one might expect to see variation in the timing of nesting and inconsistencies in
nest/chick success rates throughout a breeding season. As good parents select the most
opportune times to breed they may have greater nesting/offspring success compared to
poor parents. However, we know little about the consequences of the timing of breeding
on seasonal differences in tropical/subtropical nesting success and offspring survival.
Therefore, studies of seasonal trends in avian success in the tropics may enable us to
better address the date and parental hypotheses as they apply to tropical and subtropical
Differences in Temperate and Tropical Bird Reproductive Parameters
Life histories of birds in tropical and subtropical regions are generally characterized
by smaller clutch sizes, more nesting attempts per year, greater nest predation, better
adult survival, and longer chick development times than temperate species (Lack 1968,
Skutch 1985, Martin 1996). The windows of nesting opportunity also may be longer for
tropical compared to temperate species (Ricklefs 1968, 1976, Martin 1996). Nesting
periods may vary annually (both in duration and timing), depending on each species'
reliance on seasonally available resources and/or their flexibility regarding other
environmental constraints (e.g. excessive rainfall or drought, changes in wetland
hydrology, predation pressures, prolonged winter, etc.). If tropical birds have increased
nesting opportunities due to a prolonged breeding season, then the within-season timing
of breeding may not be as important a factor in nest success and offspring survival as
demonstrated in temperate birds.
The degree of environmental predictability and variation within the tropics may
have consequences for the reproductive behavior and physiology of birds (Wikelski et al.
2000). Precision in the timing of nesting allows seasonally breeding birds to come into a
physiological state of breeding at a specific time of year, facilitating a more conservative
allocation of resources to somatic efforts during the remainder of the year (Wikelski et al.
2000, Hau 2001). The temporal control of reproduction in birds from seasonal, temperate
latitudes is well understood (Murton and Westwood 1977, Cockrem 1995, Hau 2001). In
general, temperate birds apparently time their breeding to coincide with the period of
most abundant resources (Lack 1968, Perrins 1970, Martin 1987). If temperate
environments are more predictable in the timing of appropriate ecological conditions for
nesting than those of the tropics and subtropics, it would be beneficial for higher latitude
birds to time their breeding efforts precisely for the window of high resource allocation.
This display of timed breeding is evident in the arctic-nesting Lapland Longspur
(Calcarius lapponicus), in which the entire population arrives, establishes territories,
courts, and nests within a few days (Hunt et al. 1995). Synchronizing breeding in this
predictable, arctic environment with the period of high food availability may allow for
greater nesting success and offspring survival in Lapland Longspurs.
In contrast, if the window of suitable breeding were narrow, but unpredictable in
time, opportunistic breeders would need to maintain a state of reproductive readiness for
prolonged periods of time (Hahn 1998, Wikelski et al. 2000). Maintaining a state of
reproductive readiness may be energetically demanding (Murton and Westwood 1977),
making this a costly strategy for birds breeding in unpredictable environments.
The tropics may be less seasonal than temperate zones (Ashmole 1963, Martin
1996). Decreased seasonality in tropical and subtropical areas may lower food
productivity generally or extend the period of productivity. Food acquisition also may be
more difficult for tropical and subtropical species compared to temperate ones because of
reduced food density, or because of increased competition (Ashmole 1963). Ashmole
(1963) hypothesized that a decreased seasonality in the tropics could lead to increased
competition and niche specialization among birds. However, species in tropical areas
that experience seasonality in the form of marked wet/dry periods have been shown to
breed as seasonally as temperate species (Wikelski et al. 2000) and, in some cases, form
non-competitive niches (Ahumada 2001).
In Panama, Wikelski et al. (2000) considered the consistent between-year breeding
patterns of Spotted Antbirds (Hylophylax n. naevioides) an indication that tropical
seasons are as predictable for birds in that region as for temperate birds. Indeed, the
predictability of Spotted Antbirds' environment (-70%) was highly comparable to central
California White-crowned Sparrows (Zonotrichia leucophrys nuttalli) (-72%) and for
Song Sparrows (Zonotrichia melodia morphna) (-77%) in western Washington
(Wikelski et al. 2000). Spotted Antbirds also displayed a similar seasonal pattern of
gonad regression and recrudescence as many temperate bird species, suggesting that
Spotted Antbirds may follow a similar seasonal breeding cycle.
Less is known about the factors that influence the predictability of tropical
environments as they concern avian reproductive decisions (Hau 2001, Ahumada 2001).
Food availability is probably important, especially in environments with pronounced
wet/dry seasons, and there is evidence that some Neotropical species exhibit a distinct
breeding season (Poulin et al. 1992). However, in tropical habitats with less distinct
wet/dry seasons, food (Skutch 1950), nest predation (Young 1994a), and climate (Skutch
1950) may be important in determining when birds breed, and some species in these
environments have been shown to breed year-round (Skutch 1950, Ahumada 2001).
However, the breadth of the foraging niche also may influence the degree of
dependence of nesting timing and success on seasonal food pulses. Congeneric Buff-
breasted (Thli, ,,t // leucotis) and Rufous-and-white (Thiy IvI/h,, rufalbus) Wrens in a
dry forest of northeast Colombia may both use seasonal rains as a cue for the timing of
reproduction (Ahumada 2001). The dry Colombian forest is characterized by annual
unpredictability in the onset of the rainy season and pulses of food abundance, conditions
that could present consistent, annual uncertainties in the reproductive decisions of birds
in the area. Rainfall levels influenced arthropod abundance from year-to-year and the
two wrens differed in foraging heights and food choice. Buff-breasted Wrens foraged in
the leaf-litter, which displayed no significant change in arthropod abundance between wet
and dry seasons. Rufous-and-white Wrens foraged on understory arthropods, which
varied in abundance between seasons. The less seasonal and more predictable food source
for Buff-breasted Wrens may explain the shorter nesting period and later nest initiations
observed in Buff-breasted Wrens than for Rufous-and-white Wrens (Ahumada 2001).
Yet, some factor other than food abundance may be acting upon Buff-breasted Wrens to
shorten their nesting period.
Currently, most evidence supports the hypothesis that temperate regions are
characterized by more predictable and less variable seasonal environmental conditions
that, in turn, may allow some birds breeding in these areas to time their reproductive
events precisely for the period of greatest resource abundance. Such generalizations are
more difficult to discern for the tropics and subtropics. Ashmole (1963) may be correct
in his assessment that a less seasonal tropical habitat leads to the increased niche
specialization seen in the tropics. Some evidence suggests that species occupying
individual niches within local tropical communities may display different reproductive
strategies, even when exposed to the same environmental pressures (see Ahumada 2001).
Therefore, the well-studied examples of temperate birds seem to represent only one end
of the continuum of seasonality and predictability of nesting conditions.
It is possible that the tropics and subtropics may display a greater variation in
seasonality and predictability in the timing of nesting events and ecological conditions,
but few studies of seasonal change in tropical/subtropical avian reproductive parameters
exist. To better understand the effects of environmental predictability on the productivity
and nesting success of birds generally, we therefore need to study the seasonal nesting
success of species from a diversity of locations, especially those that represent varying
degrees of environmental predictability within the tropics.
Subtropical South Florida and the Everglades
In the subtropical Everglades of south Florida, we know that this pattern of
decreasing nest success over a season holds for Wood Storks (Mycteria americana)
(Kushlan et al. 1975, Kushlan and Frohring 1986, Frederick and Collopy 1989a, Ogden
1994). In the case of storks, the primary mechanism is obvious-late-nesting storks must
raise their young during the onset of the rainy season, when prey becomes dispersed and
less available during rising water (Ogden 1994). This situation is similar to temperate
nesting birds that face a sharp decline in availability of food with the approach of winter.
However, since wading birds vary in their foraging techniques and prey selection
(Kahl 1972, Kushlan 1976, 1977, 1979) the stork model may not be relevant for other
species of wading birds when considering the source of seasonal limitation. Different
species of wading birds also can vary in their response to foraging conditions. Certain
species (Wood Storks, White Ibises Eudocimus albus, and Snowy Egrets Egretta thula)
of wading birds' feeding strategies show a higher dependence on high densities of prey in
shallow water, giving up and searching for new foraging locations sooner than other
species (Glossy Ibises Plegadisfalcinellus, Great Egrets Casmerodius alba, Tricolored
Herons Egretta tricolor, Great Blue Herons Ardea herodias, Little Blue Herons Egretta
caerulea) (Gawlik 2002b). Wood Storks and White Ibises are primarily tactile feeders
(Kushlan 1977, 1979) and dispersion of prey as water depth increases may have a more
profound effect their foraging ecology compared to the more visual foraging methods
utilized by other wading birds (Kushlan 1976). It follows from this difference in foraging
ecology between species of wading birds that changes in hydrological regimes may affect
some species' life history strategies (including reproductive parameters) to a greater
degree than others.
Somewhat in contrast to Wood Storks, Morrison (1999) found that Crested
Caracaras (Caracaraplancus) breeding in south-central Florida experienced varied nest
success throughout a breeding season, with the lowest nesting success associated with
early and late-season nesters. The unpredictability of several environmental factors,
especially rainfall, may have contributed to the observed nest success rates in Crested
Caracaras. Winter storms, cold fronts, and variations in the timing and magnitude of
seasonal rainfall may have affected nest success either directly hypothermicc chicks), or
indirectly as ephemeral wetland water levels varied, altering the concentration of
available food resources (Morrison 1999).
Understanding the general nature and sources of declining success over the nesting
season is relevant to the success of Everglades restoration, since the timing of nesting in
several species of wading birds may be related to seasonal hydrology (Kushlan et al.
1975, Kushlan 1979, Frederick and Collopy 1989a, Ogden 1994). Changes in hydrology
can affect water depth and the location, diversity and density of prey, which may affect
the foraging success of specific wading bird species (Powell 1987, Bildstein et al. 1990,
Gawlik 2002b). Kushlan (1979) posited that the temporal and spatial components of
White Ibis nesting events in the Everglades were associated with swiftly receding water
levels. In a study of five wading bird species in the Florida Everglades, Frederick and
Collopy (1989a) associated Great Egret nesting failure with the amount of rainfall, and a
rapid surface-water drying rate was associated with White Ibis nesting success.
White Ibises are of particular interest because they represent the majority of the
avian biomass in the Everglades (Ogden 1994) and because they display very long
reproductive windows (nest initiation ranges from January to September) (Kushlan and
Bildstein 1992). This flexible breeding schedule, coupled with the nomadic movements
of this species, may allow White Ibises to exploit favorable and spatially unpredictable
breeding conditions (Frederick and Ogden 1997). The consequences of this flexibility on
nest success, juvenile survival, and demographic recruitment in White Ibises are
Due to the lack of information on within-season patterns of tropical and subtropical
nesting productivity and the unpredictability of tropical environments, I designed a study
to characterize nesting success in a particular subtropical niche, wading birds reproducing
in the wetlands of the Everglades. In this thesis, I report findings of a study designed to
compare the reproductive parameters of a primarily subtropical and tropical-breeding
bird, the White Ibis, nesting early and late in the breeding season. I predicted that White
Ibis nesting success and offspring survival would be lower in nests initiated late versus
early in the season. I compared late and early nests within several colonies, and assumed
all nests within a colony were equally affected by environmental constraints at any
particular moment in time. I expected to see significantly more eggs surviving to
hatching, higher survival rates of nestlings to fledging, higher survival rates of fledglings
to independence, and higher first-year survival rates of juveniles for early compared to
I chose a subtropical site, south Florida, U.S.A. (Chen and Gerber 1990, Duever et
al. 1994), in which to study ibis nesting productivity in relation to nest date, because the
region is a known center of nesting abundance for the species (Kushlan and Bildstein
1992), and previous reproductive information was available for this area (Frederick and
Collopy 1988, Frederick and Collopy 1989a, Frederick 1995, Heath 2002). The primary
study area encompassed the freshwater marshes of southern Florida (Miami-Dade,
Broward, Monroe, and Palm Beach counties; Water Conservation Areas [WCAs] 2B and
3A; Arthur R. Marshall Loxahatchee National Wildlife Refuge [WCA 1]; and Everglades
National Park; see Figure 1).
The landscape is divided by a system of dikes and canals, with varying levels of
water regulation and management goals within individual compartments (Light and
Dineen 1994). These areas are seasonally inundated and dominated by extensive stands
of sawgrass (Cladiumjamaicense) and cattail (Typha angustifolia). Tree islands and
open water sloughs sporadically interrupt the sawgrass mosaic (Gunderson 1994). The
south Florida climate has a distinctive annual wet and dry season and is generally
considered to have more in common with the tropics than the more temperate regions
characterizing most of North America (Duever et al. 1994). Climate is highly variable in
space and time over the south Florida ecosystem, particularly in amounts and timing of
rainfall and the severity of wet/dry years.
After some juveniles emigrated from the primary study region late in the breeding
season, I expanded the area covered by radio-telemetry flights to include the majority of
peninsular Florida, from Key Largo north to Daytona Beach. The initiation and
frequency of flights over this expanded region depended upon the pattern and timing of
juvenile emigration and/or an increased loss of signal receptions in the primary study
area. As the breeding season progressed and more marked birds from both early and late-
hatched cohorts expanded their range, I made two flights per week in areas outside the
south Florida ecosystem. In an attempt to locate juveniles marked in 2001 during their
first winter, I flew two additional nighttime flights in February 2002 over portions of
southern and central Louisiana. Combined, these flights included locations from Baton
Rouge south to the coast and east to New Orleans (see later this section).
To monitor nest success, I first located White Ibis colonies using systematic (100%
coverage) survey flights in fixed wing aircraft and later confirmed nesting stage during
ground visits. Survey flights were performed in a Cessna 182 at an altitude of 240 meters
with east-west flight transects spaced 2.6 km apart (see Frederick et al. 2001).
Attempts were made to monitor nest success in the majority of active White Ibis colonies
(2001: 6 of 9; 2002: 5 of 8) within the study area, regardless of size. However, I
excluded colonies I could not access with an airboat or on foot, usually due to extremely
low water levels. I conducted all colony monitoring activities during one-hour visits
either in the early morning (0600-1000) or early evening (1600-2000) hours to avoid heat
stress to the birds and the nests. Previous studies of wading bird nesting productivity
used one-hour colony visits (Frederick and Collopy 1988, 1989b, Frederick 1995) and I
believe that this conservative amount of time inside a colony resulted in a minimal
amount of disturbance in this study.
When a majority of nests within a colony contained two or more eggs, I marked
nests individually with strips of numbered orange or pink surveyors flagging, usually tied
to a branch below the nest. Nests too high to reach by hand were checked using a bicycle
mirror attached to the end of a telescoping aluminum pole. In small colonies (< 100
nests), all nests were marked and monitored. If the colony was large, all nests within two
meters of transects through the colony were marked. To avoid edge effects, the first
transect began at the edge of the colony, progressed toward the area of densest nesting,
and continued until it reached the center of the colony or a point at which the density of
nests declined markedly. If time allowed, I marked nests along additional transects, each
beginning -10 meters away from the end of the previous transect (i.e. either inside the
colony or along the edge) and parallel to the first transect. The one-hour visitation limit
determined the total number of nests marked per colony.
Marked nests were monitored through repeated visits to a colony every five days
during incubation and every three days after hatching. At 14 days of age White Ibis
nestlings became highly mobile, and it was not possible for me to associate chicks with
specific nest sites. My concern with colony disturbance at this time was that younger
birds (< 14 days) in the colony might not be able to return to the nest on their own.
Therefore, I stopped individual nest monitoring when, for each marked nest, at least one
nestling reached 14 days of age or when the nest failed. Colony visits stopped once I
determined the fate of all marked nests in a colony. Some colonies (5 of 9 over both
years of study) displayed multiple nesting pulses over a breeding season, and I marked
and monitored nests associated with these pulses in nesting effort.
During every visit to each nest I recorded the nest contents and any evidence of
abandonment or failure. I used the following criteria to determine nest abandonment: all
eggs in the nest were cold when touched by bare hands, all eggs or nestlings were
missing from the nest (with no evidence of predation), the eggs were intact on the ground
or chicks were dead outside the nest (with no evidence of predation), or the nest was
entirely missing or partially dismantled. Efforts were made to determine if nest contents
had been scavenged (post-abandonment) or depredated based on the presence of
mammalian feces and/or the destruction pattern of broken eggs.
To monitor chick growth I measured the mass, wing chord, tarsus, and bill length
for as many first-hatched chicks (considered the largest chick if hatching order was
unknown) in as many marked nests as could be monitored during the one-hour period
allotted for colony visits. I marked first-hatched chicks on the leg above the carpal joint
with a non-toxic red paint and efforts were made to obtain repeated measurements (see
above) on these individuals during subsequent colony visits.
Hatchability of eggs (proportion of eggs hatching divided by proportion of eggs
surviving to the date of hatch) is an indicator of eggs that fail to hatch due to embryonic
death, infertility, or poor attendance by adults. Only nests that hatched at least one chick
were used in estimating hatchability.
I estimated clutch size data from nests found during the first six days after the
laying of the first egg in order to minimize error associated with partial clutch losses
early in the incubation stage. The nest initiation date was often unknown due to the
spacing of colony visits. Therefore, the initiation date for nests found while eggs were
still being laid were backdated using a two-day interval between egg depositions. Nests
found after the completion of egg laying were backdated using the hatch date of the
nest's first young. I did not attempt to estimate the initiation date at nests for which
neither the egg laying or hatch dates were known.
I considered nest success as the probability of any initiated nest producing at least
one young to 14 days of age, at which point young were capable of escape by walking. I
estimated nest success using Mayfield's (Mayfield 1961, 1975) method. Because
survival rates may change with behavior of parents and nest content type, I estimated nest
survival rates separately for the incubation (lay date of the first egg to 20 days after the
laying of the first egg, 21 days total) and nestling (1-14 days of chick age) periods. The
spacing of colony visits generally precluded determination of the actual day on which a
nest failed. In the absence of evidence (freshly preyed upon or scavenged eggs, stage of
chick decay) to accurately date a nest failure between visits, I used the midpoint of the
interval between visits (including half-days) as the point of failure. An overall nest
success probability was calculated by combining the period-specific rates (Hensler and
Nichols 1981). Nest success estimates given are only through the period that the young
spent in the nest since I could not monitor success after 14 days of age for juveniles not
Defining Early and Late Parts of the Season
To investigate the effect of date of nesting on nest success, I compared the success
of nests initiated early with the success of those initiated late in the nesting season. For
each year, I obtained the median nest initiation date for all marked nests for which I could
determine an accurate initiation date (see above). I placed a two-week buffer around
each year's median nest initiation date, creating a minimum 15-day buffer period
between early and late-initiated nests. I considered early-season nests to be any nest
initiated at least one week prior to the median nest initiation date (pre-buffer), and late-
season nests to be any nest initiated at least one week after the median nest initiation date
(post-buffer). "Middle" nests were any nests initiated during the 15 day buffer period. I
used a Z-test to compare Mayfield nest success rates between time periods and between
years (Hensler 1981, Hensler and Nichols 1985).
To monitor the survival of fledged juveniles (> 14 days of age) I re-entered study
colonies when the majority of first-hatched young in a colony were at least 18 22 days
of age. At this age they were considered large enough to return safely to the nest by
themselves and to carry a radio transmitter. My goal was to annually capture and radio-
mark 70 juveniles, 35 early-hatched and 35 late-hatched. Radio-marked juveniles were
selected from any colony included in the nest success portion of the study that year, and I
attempted to mark juveniles from all monitored colonies that fledged juveniles (7 of 13
colonies over both years of study). Up to twelve people entered a colony simultaneously
and captured juveniles by corralling them in trees and hand-grabbing individuals. Radio-
marked juveniles were from the same colony in which nests were marked, but were not
necessarily associated with any marked nests in that colony.
I initially inspected captured birds to determine if they had sufficient mid-back
feather growth to allow for transmitter attachment and whether they had a large enough
mass to carry the transmitter safely. For each bird selected for radio-marking, I recorded
mass and length of tarsus, bill and wing chord; collected scapular feather samples (5-8
feathers, including growing feathers if necessary) for analysis of mercury content, and
blood samples for sexing; and fitted each individual with a 17 gram, 6-volt radio
transmitter (American Wildlife Enterprises, Monticello, FL; see description below). I
attached a wrap-around, plastic, uniquely numbered colorband (National Band and Tag
Co., Newport, KY) to the leg (above the carpal joint) of radio-marked juveniles for easy
identification during subsequent visits to the colony.
I collected blood samples from a prick to the brachial vein with a 27-gauge needle.
One drop of blood was collected with a capillary tube and the tube transferred to a vial
pre-filled with -70% ethanol. Samples were sexed by PE Celera AgGen (Palo Alto, CA).
Radio transmitters were within 3-5% of a juvenile's total body mass and contained a
mortality sensor that activated if the transmitter remained motionless for approximately
18 hours. I monitored radio-tagged juveniles through both aerial and ground telemetry
tracking (see description below) until a bird either died (N = 25), prematurely dropped its
transmitter (N = 1), or the study ended.
Transmitter Harness Design and Attachment
Heath (2002) used a figure-8 leg-loop Teflon ribbon harness to attach transmitters
to adult White Ibises. In order to monitor the survival of fledged juvenile White Ibises I
needed a method of transmitter attachment that would allow for the continued growth of
the birds. Juvenile White Ibises reach 90% of their final weight by 30 days of age
(Kushlan 1977) and walk away from the nest for extended periods of time at
approximately 28 days of age (nest fledge date, juveniles are not independent from their
parents or the colony at this age) (Kushlan and Bildstein 1992). However, I could not
capture juveniles after about 21 days of age, a point at which the birds were still growing.
I modified the harness used with adult White Ibises and designed an expandable
figure-8 leg loop harness to attach 122 transmitters to juvenile ibises. Teflon ribbon (6.4
mm width) leg loops were precut to the average adult ibis harness setting (Heath,
unpublished data). I loosely stitched a 6.5 mm knit polyester elastic (56% polyester, 44%
rubber) thread into the middle of each entire leg loop. Before securing the elastic thread
with a knot, I pulled it taut enough to form the Teflon ribbon into small bundles,
"accordion style," formed by holding onto the free end of the elastic and pushing the
Teflon loop up against the body of the transmitter. This significantly reduced each leg
loop's size, actually pulling the loops closer into the body when placed on a juvenile bird,
therefore reducing the chance of a bill or leg becoming entangled in the harness. The low
tension in the elastic allowed for the expansion of the harness to the average adult White
Ibis setting as the juvenile grew.
I cut a chiffon patch into the shape of the bottom of the transmitter leaving an extra
2-3 mm around the edges. Using a fast-drying epoxy, I attached the chiffon patch to the
bottom of the transmitter. The Teflon ribbon loops were sewn together with cotton
thread at the point of the adult setting, providing a weak link that would allow the
transmitter to eventually detach when the cotton thread rotted through. Transmitters
placed on adult White Ibises have been retained for over 18 months (Heath 2002).
To attach the transmitter I looped the harness around each leg and across the bird's
back, allowing the transmitter to rest anterior to the preen gland. I used fast-drying epoxy
to attach the chiffon patch on the transmitter to the juvenile's back feathers. Care was
taken not to place epoxy directly on the bird's skin. This method of attachment
successfully kept the transmitter fixed in place on the bird's back until the juvenile grew
into the adult setting of the leg loops.
Tracking Radio-Tagged Birds
I attempted to locate all radio-tagged birds through aerial and ground telemetry
tracking on a regular basis, dependent on the stage of the colony (majority of marked
birds pre-independence or independent from the natal colony). Once located, I recorded
the bird's status (either alive or dead according to the mortality sensor), marked its
location using a GPS unit (Garmin GPS 12), and tried to visually locate the bird. In order
to determine cause of death, I attempted to recover all birds (or transmitters) broadcasting
a mortality code as soon as possible after receiving a mortality signal.
When possible, I located marked juveniles from the ground until they were
independent from the natal colony. I considered a marked bird fledged from its natal
colony if I did not receive any signals from that individual within the colony on two
successive visits. Until all birds were independent from a colony I monitored for signals
at least bi-weekly. Once all marked birds were independent from a colony, I only flew
over that colony if the transect route crossed that area.
To estimate the age at which birds became independent from the colony, I
considered each radio-tagged juvenile to be 20 days of age when radio-marked. I added
to this age the additional number of days until that particular bird was not found on a
telemetry monitoring flight over its respective colony (see description above). I then
used the midpoint between first date missing and the last day found in the colony as the
independence date, unless the actual independence date was known (e.g. via daily ground
telemetry monitoring of a colony).
I performed systematic aerial searches for marked birds using radio-telemetry flight
transects spaced 7-9 km apart and flown at an altitude of 900-950 m above ground with a
ground speed of 161 km/h. Most telemetry flights were 4-5 hours in duration and
consisted of flying directly over all colonies containing marked birds, followed by flying
transects over selected areas in all Water Conservation Areas, Everglades National Park,
Big Cypress National Preserve, and the Everglades Agricultural Area south of Lake
Okeechobee. Once juveniles began dispersing from a colony I made at least two
telemetry flights per week, often in addition to continued ground monitoring of a colony.
Efforts were made to fly over all of the mentioned areas every two weeks; however, I
concentrated the majority of flights over areas with standing water or that were known
areas of dense wading bird foraging activities. As the season progressed and marked
birds began leaving the south Florida ecosystem, I flew along the coastal zones as far
north as Cedar Key and Daytona Beach, Florida, and in central Florida from Avon Park
to the southern tip of Lake Okeechobee.
From December 2001 to March 2002, I attempted to locate radio-marked
individuals by monitoring large (> 100 birds) White Ibis winter roost sites within
peninsular Florida. Roost monitoring occurred from one hour before sunset until 30
minutes to one hour after sunset and included both inland and coastal roosting locations.
In addition, I conducted two aerial nighttime roost-monitoring flights over portions of
southern and central Louisiana, from Baton Rouge south to the coast and east to New
Orleans. I selected flight areas in Louisiana based on high-density (> 1000 individuals)
locations of White Ibises counted during the Audubon Christmas bird counts in
December 2001 (National Audubon Society 2002).
Statistical tests were conducted using SPSS software version 10.1 and SAS
software version 8. I report descriptive statistics as mean + standard error unless
otherwise noted. Nonparametric tests were used (Hollander and Wolfe 1999) if
requirements for parametric tests could not be met. Comparisons of clutch sizes and nest
abandonment between early and late-nesting ibises were made using a Wilcoxon-Mann
Whitney test. I used a chi-square test of independence to compare the hatchability of
eggs between early and late nest periods.
I estimated survival of radio-tagged juveniles using the Kaplan-Meier product limit
estimator (Kaplan and Meier 1958) with a staggered entry design (Pollock et al. 1989).
Birds were censored from the study if I did not locate that individual at least once during
every 30-day period post-independence. Censored birds found later in the study were re-
entered as new additions to the population (not "back-logged" to the individual's last
known resight point). I divided juvenile survival into four distinct time periods: survival
until independence from the colony and 30, 60, and 90 days post-independence. A two-
sample test for censored data (Mantel log-rank) was used to compare survival
probabilities between early and late nesting periods for each year (Pollock et al. 1989,
Hollander and Wolfe 1999).
I marked and monitored 570 White Ibis nests between 2 March and 7 June 2001
and 790 nests between 20 March and 11 June 2002 (Table 1). These nests represented
approximately 3.3% of the total White Ibis nests initiated in the Everglades as estimated
by aerial surveys in 2001 (Gawlik 2001) and 2.4% of the total in 2002 (Gawlik 2002).
The difference in marked nest initiations between early and late-nesting ibises ranged
from 19 days (number of days between the latest, early-period nest and the first, late-
period nest) and 83 days (number of days between the first, early-period nest and the
latest, late-period nest) in 2001 and from 15-62 days in 2002 (Figure 2). Differences in
the observed range of nest initiation dates were due to the earlier onset of nesting in 2001
compared to 2002 (see Figure 2) and the prolonged 2001 nesting season compared to
Comparison of Reproductive Parameters In Early and Late Nests
Mean clutch size was significantly larger in late compared to early-nesting ibises in
2002 (U = 21706, Z = -2.106, P = 0.035) (Table 2). The observed mean clutch size for
each year (2001: 2.50 + 0.04 and 2002: 2.56 0.03) was near the middle of the range for
the species (Table 3). Hatchability (the percent of available eggs at the time of hatching
that actually hatched) of eggs from monitored nests (all clutch sizes combined) was not
significantly different for early and late-nesting birds in either study year (2001: X =
0.90, P = 0.344 and 2002: X = 2.21, P = 0.137, Table 2). The overall hatchability in
2001 (0.854) and 2002 (0.856) for White Ibises in the Everglades system was the lowest
recorded since 1986 (Table 4).
Table 1. Numbers of White Ibis nests marked by nest period and colony in 2001 and
Colony Nest Period 2001 2002
Alley Northa Early 0 246
Big Ponda Late 42 0
Hidden Middle 0 21
Late 0 92
L-67a Late 78 0
Lox 70b Early 150 85
Middle 0 53
Lox 99b Middle 0 57
Late 0 88
Lox 111b Early 203 0
Tamiami West' Early 28 0
Late 0 148
2B Melaleucad Early 4 0
Middle 33 0
Late 32 0
a) Water Conservation Area 3A
b) A.R.M. Loxahatchee National Wildlife Refuge
c) Everglades National Park
d) Water Conservation Area 2B
Table 2. Mean clutch size and hatchability (the percent of available eggs at the time of
hatching that actually hatched) comparisons from marked White Ibis nests.
Error is standard error.
Early Late Early Late
Clutch 2.52 + 0.04 2.59 + 0.08 2.49 + 0.04* 2.62 0.04*
N 197 49 183 265
Hatchability 0.82 + 0.03 0.90 + 0.05 0.88 0.02 0.83 0.02
N 53 10 176 212
*Mann-Whitney U = 21706, Z = -2.106, P = 0.035.
Table 3. Comparison of mean White Ibis clutch sizes and Mayfield nesting success estimates in this study with those from studies in
other years and locations.
Frederick and Collopy 1988
Frederick and Collopy 1988
a) Frederick and Collopy (1988)
b) Frederick (1995)
Coastal North Carolina
South Florida Lakes
interiorr Central Florida
Coastal South Carolina
Year Mean S.D. N Mayfield Nest Success (%)
Table 4. Comparison of White Ibis hatchability from marked nests in 2001 and 2002
with marked nests from WCA 3 in previous years.
a) Frederick and Collopy (1988)
b) Frederick (1995)
Mayfield Nest Success Estimates
Overall nest success estimates (probability of a nest producing at least one chick to
fledging, 14 days old) were significantly higher in 2002 compared to 2001 (Z = -19.159,
P << 0.0002) (Table 5). Early-nesters had significantly higher nesting success compared
to late-nesters in 2001 (Z = 2.64, P = 0.004, Table 5), while in 2002, late-nesters had
significantly higher nesting success compared to early-nesters (Z = -2.91, P = 0.002,
Table 5). Middle-nesting ibises displayed significantly higher nest success rates
compared to either early or late-nesting ibises in both years of study (Table 5).
Using data from two previous White Ibis nesting success studies in WCA 3 (1986
and 1987, see Frederick and Collopy 1988) I determined the early and late-nesting
demarcation dates for each year and computed Mayfield success estimates for all marked
nests. In both 1986 and 1987, early-nesters had significantly greater nesting success than
late-nesters (1986: Z = 13.231, P << 0.0002 and 1987: Z = 13.577, P << 0.0002; Figure
3). Late-nesting ibises in both years showed particularly low nesting success versus early
nesting ibises when compared to nest success patterns in 2001-2002 (Figure 3). The
Table 5. Mayfield estimates of nest success rates for White Ibis nests monitored in 2001 and 2002. Number of young fledged
is from actual observations.
Number of Successful Nests
Number of Incubation Nest Days
Survival for Incubation Period
Number of Successful Nests
Number of Nestling Nest Days
Survival for Nestling Period
Overall Nesting Success
Number of Young Fledged
Early versus Late
Early versus Middle
Late versus Middle
2001 versus 2002
Z = -2.907
P = 0.0019
P < 0.0002
P < 0.0002
Z =-19.159 P << 0.0002
median nest initiation date for 1986 and 1987 occurred during the last week of May (26
May and 29 May, respectively, Figure 2) compared to the middle of April in 2001 and
2002 (12 April and 8 April, respectively, Figure 2). The late-May start of late-nesting
ibises observed in 1986 resulted in the majority of late-nesters starting and/or incubating
nests during the onset of the rainy season for that year (1st week of June). While it is
likely that the increased number of large storms (> 2.54 cm of rain/day) associated with
the beginning of the rainy season and the subsequent rise in water levels surrounding the
colony caused the large-scale abandonment observed in 1986 (Frederick and Collopy
1988), I observed high nest success (> 63.5%) from late-nesters in 2002 even after the
start of the rainy season (last week of May) in that year (Figure 4). When compared to
nesting success from previous years in WCA 3, 2001 was at the extreme low end of the
scale for the species, while 2002 was more representative of previous years (Figure 5,
Nest Abandonment and Predation
I observed some degree of nest abandonment in all the locations within all study
colonies in both years. Abandonment appeared to be independent of island structure,
density of nesting ibises, or frequency of researcher visits. Very few nests showed
evidence of mammalian predation (Table 6). Therefore, I considered the majority nests
found empty during a colony visit to have been abandoned by one or both parental birds.
Many abandoned nests were often associated with post-abandonment scavenging
(primarily from Boat-tailed Grackels Quiscalus major). Nest failure frequently occurred
in localized areas within a colony, with several nests in close proximity to one another
failing simultaneously. Nest abandonment ranged from 33.3% (2B Melaleuca) to 100%
(L-67) in 2001 and from 37.6% (Hidden) to 69.4% (Lox 70) in 2002 (Figures 6 and 7).
In 2002, late-nesters displayed significantly less nest abandonment compared to early-
nesters (U = 50000.5, Z = -2.099, P = 0.036, Figure 8).
Table 6. Causes of nest failure in early and late-initiated nests monitored in 2001 and
Early Late Early Late
Abandoned 266 146 203 89
Mammalian Predation 2 4 3 5
Other (wind damage, researcher handling, etc.) 4 0 2 2
Total 272 150 208 96
There were no complete failures of all nests within any White Ibis colonies in 2000
(Frederick et al. 2001) or 2002 (including those in which I did not monitor nests, data
obtained through aerial surveys), compared with the complete abandonment of some (2
of 6) marked colonies in 2001. Through aerial surveys in 2001, I observed repeated
initiation of large-scale ibis nesting events (N = 9 colonies > 450 nests) followed by
complete or nearly compete abandonment (> 75%) in several colonies (N = 6).
Survival of Young Post-Fledging
I radio-tagged a total of 122 juvenile White Ibises from six colonies in 2001 and
2002 (Table 7). Age of independence for marked birds ranged from 51-70 days of age,
and the mean age at independence was about 61 days for both years (Table 7).
Table 7. Distribution of radio-marked juveniles by nest period and mean age (in days) at
independence for all marked birds that survived to independence.
Early Late Combined Early Late Combined
N 29 24 53 35 34 69
Age 60.8 1.5 60.1 + 1.3 60.5 1.0 62.2 0.8 58.4 0.7 60.7 0.6
I experienced very different location-rates of radio-marked birds' signals between
2001 and 2002. In 2001, only 16 of 46 (34.7%) birds that survived to independence were
located at least once outside their natal colony compared to 43 of 57 (75.4%) juveniles in
2002 (Appendix B). Several reasons may partially account for this difference in the
location-rates of transmitter signals including transmitter and/or receiver malfunctions, a
poor ability to receive mortality signals, and faster emigration rates of juveniles to areas
outside central and southern Florida in 2001 compared to 2002.
Due to the simultaneous use of two receivers on numerous telemetry flights in both
years of the study and my ability to locate birds' signals during these flights, it seems
unlikely that receiver malfunction caused such a large discrepancy in resight rates. A
transmitter's frequency can shift or change, especially during its first six weeks in use,
potentially altering the frequency enough to mask detection. To avoid this problem I
activated transmitters a minimum of two weeks prior to attachment and tuned each
transmitter on every receiver the night before deployment. This allowed me to obtain a
transmitter's stable frequency before attachment. During the first month post-attachment,
all birds carrying transmitters remained in their natal colony, giving me the opportunity to
further tune transmitter frequencies if they continued to shift during this period. It is
therefore unlikely that the changing of transmitter frequencies caused the discrepancy in
resight rates; however, I could not entirely rule out the possibility of complete transmitter
Mortality signals can be difficult to receive if they are not located soon after death
occurs. Carcasses, along with the transmitter, can be deposited in trees, on open or
densely covered ground, under ground, or under water. Depending on the final location,
it is possible for signals to become blocked by the surrounding environment, with
transmitters underwater and/or underground more difficult to locate than those in more
open environments (see Table 8). Scavengers also may move the radio, thus resetting it
to normal mode and possibly preventing the detection of actual mortality some of the
I conducted an experiment to determine the best flight altitude and transect spacing
during telemetry flights and my ability to receive signals from the ground. I spaced four
pairs of transmitters two meters apart in each of four positions: suspended 1.5 m above
ground on a wooden post out in the open, suspended 1.5 m above ground on a wooden
post in dense vegetation, buried -5 cm underground, and -0.5 m underwater.
To test reception distance from the ground I gradually moved further away (directly
out into the wetland with an unimpeded view) from the transmitters until I lost reception.
Aerial reception consisted of testing signals from 300 m to 1200 m in elevation (in -150
m intervals) at increasing distances away from the point of origin (from 0 m 9.5 km).
Receptions distances varied depending on both elevation and direct distance from the
transmitter (Table 8). Based on these estimations and previous experience, I believe that
our transect spacing and altitude were sufficient for locating live birds. No possible
coverage, however, could guarantee successful location of all mortality signals every
Table 8. Maximum reception distances (km) of transmitter signals from both ground and
Altitude (m) Suspended, unblocked Suspended, blocked Underground Underwater
300 4.8 4.8 1.6 3.2
460 5.6 5.6 1.6 3.2
610 8.8 7.2 1.6 3.2
760 9.6 8.8 1.6 3.2
915 9.6 9.6 1.6 3.2
1070 9.6 8.8 1.6 3.2
1220 11.2 8 1.6 3.2
Ground 6.4 4 1.2 2.4
In both years of the study my telemetry tracking efforts covered the same areas (see
Methods section) with approximately the same intensity and at similar times in the
respective breeding cycles, using the exact same methods. The only exception was more
intense coverage of southeast Florida landfills, including the Palm Beach County Waste
Facility where I located birds marked in both study years in 2002. In addition, I found an
increased number of mortalities outside the colonies in 2002 (7 of 19 mortalities)
compared to 2001 (0 of 4 mortalities) using these same methods, weakening the
hypothesis that I could not locate mortalities. However, this does not preclude the fact
that there may be mortalities within the study area in both years that went undetected.
Therefore, the difference in signal location rates between years is probably best explained
by a combination of a higher rate of undetectable mortalities and a faster emigration rate
of juveniles out of central and southern Florida in 2001 compared to 2002.
While some juveniles flew only a short distance (< 5 km) upon independence from
a colony, there was also the potential for long distance emigration (> 120 km) within the
first week post-independence. The majority of marked birds' locations were fairly evenly
spread between wetland, landfill, and agricultural sites in 2001 and wetland and
agricultural areas in 2002 (Appendix B).
In both study years, early-hatched juvenile ibises displayed significantly higher
survival rates compared to late-hatched juveniles at least until independence from the
colony (2001: Mantel = 2.33, P = 0.0087 and 2002: Mantel = 3.48, P < 0.0002; Figures 9
and 10). Due to the low resight rates of marked individuals in 2001 I did not estimate
survival beyond independence for that year. Juvenile survival rates remained
significantly higher in 2002 for early-hatched compared to late-hatched birds at 30, 60,
and 90 days post-independence (Figure 10).
I found no significant differences between the survival to independence of early-
hatched birds in 2001 (96.6 %) compared to early-hatched birds in 2002 (97.1%) (Mantel
= 0.78, P = 0.2177). However, late-hatched juveniles in 2001 (79.2%) showed
significantly higher survival rates compared to late-hatched juveniles in 2002 (67.7%)
(Mantel = 2.40, P = 0.0082), at least until independence. When comparing the overall
survival to independence of all marked birds in 2001 (88.7%) to those in 2002 (82.6%),
birds hatched in 2001 had a significantly higher survival rate compared to those hatched
in 2002 (Mantel = 2.53, P = 0.0057).
The 2001 and 2002 White Ibis breeding seasons occurred under somewhat different
environmental conditions that may have resulted in the observed reversal in the pattern of
ibis nesting success between years. In particular, the very different breeding season
hydrology patterns that existed over large portions of the WCAs may have influenced the
timing of available food resources and access by mammalian predators, thus potentially
affecting ibis reproductive success. Nearly twice the number of White Ibis nests were
initiated in the Everglades in 2002 (32,573) compared to 2001 (17,262) (Gawlik 2001,
2002a). Numbers of nesting White Ibises in the Everglades are known to vary widely
during extremely wet or dry years, partially due to regional shifts in nesting locations
(Ogden et al. 1980, Ogden 1994) and partially due to an association of "supernormal"
wading bird nesting events closely following a drought year (Frederick and Ogden 2001).
The drying of the marsh surface in the nesting season of 2001 was much more
pronounced than in 2002, resulting in the loss of surface water surrounding all study
colonies. The more rapidly drying surface water in 2001 may have influenced the
availability of White Ibis food resources by increasing the concentration of prey as
ephemeral pools of water shrank and grew shallower (Kushlan et al. 1975, Frederick and
Collopy 1989a, Bildstein et al. 1990). In many parts of the ecosystem however, the
drying may have been so rapid that it caused large-scale die-offs of prey that were left
without any water (personal observations from WCAs 1, 2B, and 3B in 2001). Under
such stressed environmental conditions ibises may have experienced greater difficulty
foraging for both themselves and their young as prey became more difficult to locate and
obtain. If breeding ibises did experience such harsh foraging conditions in 2001, the
result of these difficult conditions could have been one factor causing nest abandonment
in that year.
Other factors such as an increased exposure to mercury contamination may have
affected ibis nesting efforts in 2001 (Frederick et al. 2001, Heath 2002). Wetlands are at
risk to high levels of mercury contamination (Eisler 1987, Zillioux et al. 1993).
Inorganic mercury may become bound in wetland soils, converting to the more toxic
organic methyl-mercury as environmental conditions change (i.e. drought, decreased
water levels, increasing anoxia) (Zillioux et al. 1993). Bioaccumulation of mercury in
wetlands is a known potential threat to wildlife, including predatory birds (Scheuhammer
1987, Wolfe et al. 1998). Mercury's ability to increase in concentration by several orders
of magnitude from surface waters to fish is well documented (Scheuhammer 1987,
Driscoll et al. 1994) and it follows that piscivorous species, such as wading birds, feeding
on small fish and invertebrates are at a high risk of accumulating toxic levels of
methylated mercury (Jurczyk 1993). Thus, during the pronounced low water levels in
2001, an increase in mercury's conversion to methyl-mercury and a subsequent increase
in the uptake of methyl-mercury by foraging wading birds may have been expected
compared to the conditions associated with the 2002 breeding season.
Chronic levels of mercury exposure in White Ibises may interfere with the
hormonal mechanisms associated with nest attendance (Heath 2002), which could cause
nest abandonment, especially during rapidly occurring, short-term changes in
environmental conditions similar to those observed in the Everglades in 2001. However,
without evidence that mercury levels in 2001 were high enough to lead to the
abandonment of nests and significantly higher than 2002 levels, exposure to mercury
does not appear to be a likely cause of ibis nest abandonment during this study.
I am uncertain as to the cause of the lower hatchability observed in both study years
compared to previous years in the interior Everglades. While mercury contamination is a
possible explanation for low hatchability, it appears unlikely since mercury levels in the
Everglades have decreased considerably since 1994 (> 75% in Great Egrets) and
hatchability rates in earlier, higher-exposure years have not been as low as during the
years of my study (Frederick et al. 2001).
Mayfield Nest Success Estimates
The evidence from this study indicates that White Ibis nesting success in the
Everglades seems to vary unpredictably as the breeding season progresses, contrary to
my predictions and different from the typical pattern of nesting success observed in most
temperate breeding species of birds. It is interesting to note that ibis nests initiated during
the "middle" of the nesting period in 2001 and 2002 (early/mid-April) showed the highest
nesting success rates. In comparison, those ibises initiating "early" relative to the nesting
season in 1986 and 1987 (late-April/early-May) resulted in extremely high nesting
success (75.6% and 88.8%, respectively) compared to the same time period in 2001 and
2002 (4.6% and 33.4%, respectively). This pattern of high variation in nesting success at
the same time of season may be expected in an environment with conditions similar to
those typically experienced in the south Florida ecosystem. The within-season variability
in the annual timing of environmental conditions in the Everglades may have contributed
to the extreme annual differences in nest success.
It is tempting to consider the evidence presented in this study as partial
confirmation that tropical/subtropical environments have a greater degree of variability in
the timing and extent of environmental phenomena (i.e. onset of wet season, period of
greatest resource abundance). I do not believe however, that enough evidence is
available to generalize whether tropical and/or subtropical ecosystems are either as
predictable or less predictable in the timing of environmental events than temperate
systems. We also lack information regarding differences in the variability in magnitude
of ecological conditions and seasonal weather patterns between temperate and tropical
Current evidence suggests that in some tropical locations and with some species,
tropical systems can be as predictable in the timing of ecological conditions as temperate
areas (Stiles 1980, Wikelski 2000). Therefore, the tropical/temperate dichotomy may be
a poor example for predicting variation in nest productivity. Our efforts may be better
spent focusing on the difference in the magnitude of variation and predictability in the
ecological conditions supporting successful breeding events as opposed to attempts in
locating where areas with different magnitudes of variation in the timing of ecological
phenomena are located.
Even less evidence is available correlating either a seasonal or unpredictable
breeding pattern in the tropics with patterns of nesting success since few studies exist
detailing nest and post-fledging success rates in tropical and subtropical bird species.
One previous study of avian nesting success in Florida's subtropical zone found a similar
variability in nesting success as White Ibises' in this study (Morrison 1999). Morrison
suggested that the highly unpredictable environmental conditions in the central and
southern Florida ecosystem might have influenced the observed survival rates in Crested
I suggest that the varied patterns of White Ibis nesting success observed in this
study are due to the unpredictability associated with wetlands in general, regardless of
latitude, and do not represent evidence that there is greater unpredictability in
subtropical/tropical, compared to temperate environmental conditions. Like many
freshwater wetlands, the Everglades is characterized by periodic fluctuations in
hydrology which can dramatically alter plant and animal communities, fire regimes, and
ecosystem functions on an annual basis (DeAngelis and White 1994, Gunderson and
Snyder 1994, Mitsch and Gosselink 2000). Varying hydrologic patterns can also affect
wading bird use of foraging habitat directly through water depth changes (Kushlan 1977,
Kushlan 1986, Bancroft et al. 1991), which affect food availability (Powell 1987,
Bildstein et al. 1990). I suggest that, particularly in the Everglades, these hydrologic
patterns are hypervariable among years, which makes it very difficult for adult wading
birds to predict the timing of most opportune nesting and the best colony location
(Frederick and Spalding 1994).
Consistent with some other studies of avian reproductive success (Sydeman et al.
1991, Morrison 1999, Lepage et al. 2000), I found a peak in nesting success during the
middle of the nesting period in both years. However, late-nesting White Ibises' success
varied between years. In contrast, late-season declines in nesting success have been
observed in both tropical/subtropical (Morrison 1999, Ramos 2001, Olmos and Silva e
Silva 2001) and temperate (Perrins 1970, Sydeman et al. 1991, Norris 1993, Brinkhof et
al. 1993, Burger et al. 1996) studies. In both 2001 and 2002, water levels in the WCAs
approached their lowest point of the year when middle-nesting birds began hatching
chicks (1st week of May). If prey concentrations increased due to the lower water levels
at this time, middle-nesting ibises may have been able to feed their young better, resulting
in the observed higher fledging rates. The comparison of temperate and tropical studies
that considered seasonal variation in reproductive success, therefore, appears to indicate
that there is no overall pattern regarding success outside of the higher latitudes.
Nest Abandonment and Predation
Large scale nest abandonment and subsequent low nesting success have been
attributed to fast increases in the water level surrounding colonies of White Ibises
(Frederick and Collopy 1989a, Frederick and Spalding 1994), which can disperse
concentrations of prey, leading to reduced prey availability. In 1999, Frederick et al.
(2001) observed some abandonment of late-nesting White Ibises after the onset of the
rainy season; however, the majority of ibises were successful. In mid-April 2000, a
strong pulse of rains (3-day total: 159.51 mm of 164.59 mm monthly total) is believed to
have caused the widespread abandonment observed in Loxahatchee N.W.R. that year
(Frederick et al. 2001). Therefore, it is likely that the rapid rise of water levels in June
2001, which inundated ibis nests located at ground level in the L-67 colony, precipitated
the eventual complete abandonment of that colony. However, heavy rainfall and fast-
rising water levels are not necessarily a precursor to colony failure. In 2002, two late-
nesting colonies (Tamiami West and Hidden) successfully fledged a majority (TW: 76%;
Hidden: 85%) of the remaining active nest attempts in those colonies, even after the onset
of the rainy season in late May of that year.
In addition to acting as a potential cause of food dispersal through rising water
levels, the varying intensity and timing of rainfall events during the nesting stage also
may impact White Ibis nest success. Heavy or prolonged rainfall may have a greater
impact on reproductive success if associated with young (nestling) chicks compared to
chicks closer to becoming independent from the colony. Younger chicks may have an
increased vulnerability to rains and could subsequently suffer from hypothermia if they
are unable to maintain a constant body temperature. Pulses of rains in the early stages of
nesting (including young chicks) may affect a parent's choice of whether or not food will
continue to be available in sufficient quantities for the duration of the breeding cycle. If
rains early in the nesting cycle are heavy and prolonged, parents may choose to abandon
that particular nest attempt.
Human disturbance, via entering colonies, can affect nesting success, with nesters
in certain stages (i.e. courtship and egg-laying) more sensitive to disturbance than others
(Tremblay and Ellison 1979). In a study of Tricolored herons (Egretta tricolor),
Frederick and Collopy (1989b) found no difference in the effect of the frequency of visits
on five different measures of reproductive success. In my study, I entered colonies on a
regular basis only after the majority of nests were near the completion of egg-laying, and
the frequency of my visits was less than that in other studies of reproductive success
(Frederick and Collopy 1989a, 1989b, Sydeman et al. 1991, Erwin et al. 1999). Using
the same methods in both study years, I found large differences in abandonment rates
between the two years of study. This suggests at the very least that my own disturbance
was unlikely to have been the sole source of the variation between years. In addition, I
observed the greatest abandonment in 2001, a year in which a number of colonies (see
earlier) that I did not visit were abandoned. Disturbance therefore seems to be a weak
predictor of variation in nest success in my study.
While checking marked nests I did observe several nest scavenging events in Lox
70 in 2001 and in Lox 70, Lox 99, and Alley North in 2002. I believe, however, that the
majority of this scavenging occurred post-abandonment (again, primarily due to Boat-
tailed Grackles). Human disturbance has been associated with increased predation and
scavenging of nest contents (Frederick and Spalding 1994). Yet, increases in scavenging
usually occurred while researchers were in the colony. To minimize this effect, I
immediately stopped colony visits when it appeared that our presence affected the
behavior of scavengers. Because of the precautionary measures taken to avoid human
disturbances inside and outside the colony, I do not believe that my activities
significantly affected predation or scavenging rates.
Previous studies of wading birds in the Everglades (Frederick and Collopy 1989c,
Frederick and Spalding 1994) found little evidence of nest predation, especially by
mammals. In the interior wetlands of the Florida Everglades, predation accounted for
only 2.5% of nest failures among colonially nesting wading birds (N = 826) (Frederick
and Collopy 1989c). Mammalian predators may be deterred from wading bird colonies
by the presence of alligators (Alligator mississippiensis). Surprisingly low water depths
(5-10 cm) may be sufficient to severely limit mammalian predator movements and access
to wading bird colonies (Frederick and Collopy 1989c). Individual or group nest-defense
behavior is not commonly observed among Ciconiiformes, and large-scale abandonment
of nests may be the result of a few individual predators (Shields and Pamell 1986,
Frederick and Collopy 1989c). The extremely low levels of predation observed in this
study support the idea that, during my study, predation was not a significant contributor
to nest failures.
Post-Fledging Survival of Young
In spite of high survival during the nestling stage for late-hatched ibises in both
years (relative to overall survival in those years), the majority of late-hatched ibis'
mortality occurred during the nestling stage, while birds were still in their natal colony.
Some (-26%) of these pre-independence mortalities occurred during the time period
when I expected the juveniles to leave the colony (defined by age of nestling in relation
to mean age-at-independence). Considering the lower post-fledging survival of late-
hatched ibises compared to early-hatched birds in this analysis, it is possible that an
increased proportion of late-hatched ibises in both years were not fully fit to make the
transition to independence and survive the challenges of life outside parental support.
However, visual inspection of nestling growth data does not appear to support this idea.
This pattern of lower survival in late-hatched birds may have been due to poorer food
availability later in the breeding season, inattention by parents, disease, or some
combination of these factors. This transition period (prefledging to fledging) is generally
when juvenile birds of many species tend to die (Gill 2000) and the affects of this period
of increased juvenile mortality could be exacerbated by any harsh environmental
conditions that late-hatched ibises may face compared to early-hatched birds.
Based on my predictions, greater nesting success should be associated with greater
juvenile survival post-independence. However, in 2002, even though late-nesting ibises
showed significantly greater nesting success compared to early nesters, juveniles from the
early-hatched cohort displayed significantly higher post-independence survival rates
compared to late-hatched ibises. Brinkhof et al. (1997) found a seasonal decline in the
post-fledging success of European Coots, attributed to the smaller size of late-hatched
young. Other studies (e.g. Verhulst and Tinbergen 1991) suggest that competition
between early and late-hatched young may account for the observed seasonal decline in
post-fledging survival. Several explanations for the pattern of post-fledging success in
this study are possible. An abundance of available food for early-hatched ibises may
have been depleted or dispersed by the time late juveniles were independent. Another
possibility is that the increased amount of time early juveniles have to learn life skills
(such as foraging, predator avoidance, etc.), grow and increase their body mass before the
onset of the rainy season when prey become more difficult to obtain may give them an
advantage in first-year survival compared to late juveniles.
Marked juveniles appeared to favor the mosaic of wetland, agricultural, grove land,
and ranchland that dominates the non-coastal landscape southwest and west of Lake
Okeechobee. Habitat use by post-fledging ibises varied between the two years. The
smaller than expected number of signal locations of marked birds in any coastal area
and/or inland wetland areas in 2001 (30% in 2001, 48% in 2002, when only considering
signal locations >5 km from natal colony; see Appendix B) is interesting as it suggests
that juveniles were inconsistently using these locations as post-independence habitat in
those years. Anecdotal aerial observations through September of both years confirmed
low numbers of foraging wading birds in south and central Florida in 2001, while large
(> 200 individuals) groups were spread across the same area in 2002. Water levels in
2002 were visibly higher than 2001 levels throughout south and central Florida, with
large tracts of land that were dry in 2001 containing standing water in 2002 (personal
observations). This may help to explain why I observed more foraging birds in 2002
compared to 2001, and why more birds remained in these areas until later in year in 2002.
CONCLUSIONS AND FUTURE DIRECTIONS
White Ibis nesting success in the Everglades varied inconsistently with respect to
timing during the breeding season. Early-hatched juvenile ibises had a greater survival
compared to late-hatched ibises, at least until independence from the colony, and this
appeared to remain true through 90 days post-independence. Finally, contrary to my
predictions, nesting success was not necessarily a predictor of post-independence juvenile
Greater unpredictability in the timing of annual and within-season environmental
conditions in subtropical compared to temperate zones may cause the increased
inconsistency in both the timing of nesting events and the trends in nest success observed
in ibis nesting success rates. I suggest, however, that the varied patterns of White Ibis
nesting success observed in this study are due to the unpredictability associated with
wetland ecological conditions in general, regardless of latitude, not necessarily the sole
result of greater unpredictability in subtropical/tropical, compared to temperate,
environmental conditions. In either situation, environmental inconsistencies between
breeding seasons could make it difficult for adult White Ibises to select the optimal time
to initiate nesting attempts, therefore potentially compromising nest success and offspring
Direct comparisons of this research with a study of similar Ciconiiform species
from the temperate zone were not possible. In order to better elucidate any differences in
temperate and tropical/subtropical nesting success rates, more research is necessary on
species with comparable phylogeny and life histories. Especially lacking are long-term
studies monitoring the post-independence survival of juveniles. A comparison of ibis
nesting data from the northern part of their breeding range could help determine patterns
of within season nest success for this species. An in-depth study of the local climatic
differences between temperate and tropical species, how this may affect the formation of
local niches within communities, and monitoring of nesting success rates in these locales
would help determine whether tropical and subtropical areas experience greater annual
and within-season environmental predictability or if inconsistencies in reproductive
parameters are the result of adaptation to specific, local habitat constraints.
-II QO Study Colonies
A Lox 111 F L-67
B Lox 70 G Tamiami West
C Lox 99 H Hidden
0 50 Kilometers D 2B Melalueca I Big Pond
E Alley North
Figure 1. Location of monitored White Ibis colonies in 2001 and 2002. The primary
study area (monitored colonies) is darker (inset) and the area of intense aerial
telemetry monitoring is lighter shaded (inset).
......................... M middle
15-Feb 1-Mar 15-Mar 1-Apr 12-Apr 18-Apr 26-Apr 2-May 10-May 18-May 24-May 2-Jun 12-Jun 1-Jul
Figure 2. Nest initiation ranges by nesting period for marked White Ibis nests in 1986, 1987, 2001 and 2002. Vertical lines designate
Figure 3. Four year comparison of Mayfield nest success estimates of White Ibises nesting early and late in the season. Sample sizes
are denoted at the upper left corner of bar. Letters beside years indicate a significant difference between early and late-
nesting ibises in that year. a) Z = 13.231, P << 0.0002; b) Z = 13.577, P << 0.0002; c) Z = 2.641, P = 0.004; d) Z = -2.907,
P = 0.002. 1986 and 1987 data from Frederick and Collopy 1988.
S 0. 25.00
o 0.5 i
0 20.00 "
i-i ... I I I I I 0.00I i -- j-'1 -- I o _A
1986 1987 2001 2002
Figure 4. Monthly rainfall (solid line), breeding chronology (proportion of marked nests initiated each month; striped bars), and nest
failures (proportion of nest attempts that failed; empty bars) of monitored White Ibis nests in the interior Everglades.
Rainfall data was obtained from the Tamiami Trail Ranger Station (40 mile bend) rain gauge.
Figure 5. Multi-year comparison of Mayfield nesting success estimates for White Ibises in the interior Everglades.
(middle) nests for 2001 and 2002. a) Frederick and Collopy 1988. b) Frederick 1995.
Tamiami West Loxahatchee 111 Loxahatchee 70 2B Melaleuca* Big Pond L-67
Figure 6. Percent nest abandonment of White Ibis nest attempts (marked nests) in 2001. Asterisks denote colony includes both early,
middle, and late marked nests.
Figure 7. Percent of White Ibis nest initiations (marked nests) that were abandoned in 2002. Asterisks denote colony includes both
early and middle marked nests.
Figure 8. Comparison of overall nest abandonment rates at marked White Ibis nests for early and late-nesting pairs in both study years
(all colonies inclusive). Asterisks indicates significance at P = 0.036.
j E Early
Figure 9. Kaplan-Meier survival estimates for marked juvenile ibises in 2001. Asterisks indicate a significant difference between
early and late-hatched birds (Log Rank: P = 0.01).
Independence 30 days 60 days 90 days
Figure 10. Kaplan-Meier survival estimates for marked juvenile ibises in 2002. Asterisks indicate a significant difference between
early and late-hatched birds (Log Rank: all Ps < 0.0002).
LOCATIONS OF COLONIES WITH MARKED NESTS IN THIS STUDY
RADIO-TRACKING RESIGHT DATA FOR ALL MARKED BIRDS IN 2001 AND
Mean Age-at-Independence (days)
Range of Age-at-Independence
Total Surviving to Independence
Total Juveniles Located At Least Once Post-
Mean Age at Mortality
Range of Mortality ages
% Located Post-Independence
% Located in Wetland Areas
% Located in Agricultural Areas
% Located in Urban Areas
% Located in Coastal Areas
% Located in Landfill Areas
Total resights (all birds)
Appendix B. Continued
Mean Age-at-Independence (days)
Range of Age-at-Independence
Total Surviving to Independence
Total Juveniles Located At Least
Mean Age at Mortality
Range of Mortality ages
% Located Post-Independence
% Located in Wetland Areas
% Located in Agricultural Areas
% Located in Urban Areas
% Located in Coastal Areas
% Located in Landfill Areas
Total resights (all birds)
Tamiami West Hidden
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John David Semones began his environmental research and conservation advocacy
career during the summer of 1994 as a Student Conservation Association volunteer in the
Olympic National Forest, Washington. Work with Northern Spotted Owls and Marbled
Murrelets began an ongoing concentration in endangered bird research, conservation,
education, and recovery. He received a Bachelor of Science in biology from Davidson
College, Davidson, North Carolina, in 1995. John David spent his first year post-
undergraduate as an environmental educator, designing and teaching hands-on ecological
classes for the Honey Creek Environmental Education Center on the southeast coast of
Georgia. In the summer of 1996 he left Georgia for migratory bird work in Alaska,
initiating a nearly four-year stretch of field biological work in various ecosystems across
the country. He spent a year in Arizona working with the endangered Southwestern
Willow Flycatcher and 2.5 years on the Big Island of Hawaii with the Palila Restoration
Project. John David began his Master of Science degree with the University of Florida in
the fall of 2000, concentrating on wading bird reproductive success in the Everglades.
Upon completion of his degree John David plans on attending law school, focusing on
environmental and natural resources law.