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SURVIVAL, HABITAT USE, AND MOVEMENTS OF
FLEDGLING WHITE-THROATED ROBINS IN TROPICAL
AGRICULTURAL HABITATS

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EMILY BETH COHEN

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SURVIVAL, HABITAT USE, AND MOVEMENTS OF FLEDGLING WHITE-
THROATED ROBINS IN TROPICAL AGRICULTURAL HABITATS

By

Emily Beth Cohen

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Zoology

2003

ABSTRACT

SURVIVAL, HABITAT USE, AND MOVEMENTS OF FLEDGLING WHITE-
THROATED ROBINS IN TROPICAL AGRICULTURAL HABITATS

By

Emily Beth Cohen

I used radio-telemetry to study the behavior of White-throated Robins (Turdus
assimilis) during the postfledging dependent period. The study was conducted in
a mixed agricultural and forested landscape in southern Costa Rica from March
through August of 2001 and 2002. One fledgling per brood was located daily
until dispersal from the natal area. I compared survivorship, habitat use, and
movements for fledglings from nests in coffee plantations and cattle pastures.
Birds that fledged from nests in pasture moved into forest more quickly and were
more likely to survive until dispersal from their natal areas than were birds from
nests in coffee. Pasture habitat was rarely used during the postfledging period
while coffee and forest were used extensively. Fledglings that remained in
agricultural habitats were less likely to survive until dispersal than were those that
moved into forested areas. Average daily distances from the nest gradually
increased until fledglings dispersed away from their natal areas, always into
forest, and were not different for birds from pasture or coffee. While the species
can nest successfully in agricultural habitats, the use of forest positively

influenced survivorship of young during the postfledging dependent period.

To Phil and my family.
I could not have accomplished this without your help.

iii

ACKNOWLEDGEMENTS

Funding was provided by the George J. and Martha C. Wallace Endowed
Scholarship, the Association of Field Omithologists' E. Alexander Bergstrom
Award, the Michigan State University Center for Latin American and Caribbean
Studies, the Wilson Society's Paul A. Stewart Award, the Michigan State
University Chapter of Sigma Xi, the Michigan State University Graduate School,
the Michigan State University Department of Zoology, and the American
Omithologists Union's Marcia Brady Tucker Travel Award. I am thankful to
Addison Fisher for allowing me to work on his property and this project could not
have been completed without the logistical support of Fernando Castaneda at
Finca Las Alturas. To my committee, Dr. Tom Getty, Dr. Catherine Lindell, and
Dr. Scott Winterstein, thank you for sharing your knowledge and expertise during
this project. I also appreciate the extra assistance of Dr. Scott Winterstein with
the analyses. To my advisor, Dr. Catherine Lindell, thank you for your mentoring
and guidance throughout this project. I am thankful to Laura Riba Hernandez,
Gabriela Demograsso, and Sara Kaiser for their help, friendship, and support
when I needed it most. I am especially indebted to Phil Heavin for his love and

support and for his ingenuity and tireless hard work in the field .

iv

TABLE OF CONTENTS

LIST OF TABLES .................................................................................. vi
LIST OF FIGURES ............................................................................... vii
CHAPTER 1: BACKGROUND INFORMATION ............................................ 1
INTRODUCTION .......................................................................... 1
POSTFLEDGING PERIOD ............................................................. 2
STUDY SPECIES ........................................................................ 10

CHAPTER 2: SURVIVAL, HABITAT USE, AND MOVEMENTS OF
FLEDGLING WHITE-THROATED ROBINS IN TROPICAL

AGRICULTURAL HABITATS .................................................................. 12
ABSTRACT ............................................................................... 12
INTRODUCTION ........................................................................ 14
OBJECTIVES ............................................................................. 17
METHODS ................................................................................. 18

Study Site ......................................................................... 18

Nest Searching and Radio Attachment .................................... 19

Data Collection .................................................................. 21
Statistical Analyses ............................................................ 22
RESULTS ................................................................................. 25
Survival ........................................................................... 25
Habitat Use ....................................................................... 31
Movements ....................................................................... 33
DISCUSSION ............................................................................. 39
Survival ............................................................................ 39
Habitat Use ....................................................................... 41
Movements ....................................................................... 42
Conservation Implications .................................................... 44
LITERATURE CITED ............................................................................ 46

LIST OF TABLES

Table 1. Kaplan-Meier survival rates during the first three weeks out of the
nest for fledglings from nests in pasture and coffee habitats. Survival rates
are calculated only on days when mortalities were detected .......................... 26

Table 2. Kaplan-Meier survival rates during the first three weeks out of the

nest for birds that remained in agricultural habitat (coffee or pasture) and

birds that moved into forest habitat. Survival rates are calculated only on

days when mortalities were detected ........................................................ 28

Table 3. Locations in each habitat type, prior to dispersal, for fledglings
from nests in coffee and pasture that survived and were tracked until
dispersal from their natal areas ............................................................... 32

vi

LIST OF FIGURES

Figure 1. Map of the study site with the locations and habitat types
of the nests from which fledglings were tracked. Images in this thesis
are presented in color ............................................................................ 20

Figure 2. Comparison of survival probability curves for fledglings from nests

in coffee versus pasture during the first three weeks out of the nest. Daily
survival probabilities are cumulative (vertical bars represent SE) and are
calculated using a Kaplan-Meier procedure ............................................... 27

Figure 3. Survival probability curves for fledglings that remained within
agricultural habitats and fledglings that moved into forest habitat during the

first three weeks out of the nest. Daily survival probabilities are cumulative
(vertical bars represent SE) and are calculated using a Kaplan-Meier

procedure ........................................................................................... 29

Figure 4. Average daily distances from nests at successive ages prior to
dispersal. Bars represent SD and sample sizes are in parentheses ............... 34

Figure 5. Example movements of two fledglings, U3-01 (yellow) and

T-O2 (blue), illustrating the typical movement pattern of remaining

near their nests (in red) then making a long dispersal movement

always into forested habitat. .................................................................. 37

Figure 6. Correlation between fledging date and age at dispersal from the
natal area (r9,= -0.51, n = 28, P< 0.01) ..................................................... 38

vii

CHAPTER 1
BACKGROUND INFORMATION

INTRODUCTION

The postfledging period, or the time after fledging the nest when young
birds continue to be dependent on their parents, has received comparatively little
attention despite the fact that it may be longer and require greater parental
investment than the nesting period. Most studies of breeding passerines have
concentrated solely on the nesting period due to the difficulty in monitoring young
birds after they leave the nest. However, studies of the nesting period alone
cannot accurately assess parental investment, reproductive output, or breeding
habitat requirements. In this introduction, I will review the importance of the
postfledging period for passerifonn species and discuss how research on this
period is essential for understanding several aspects of avian biology including
brood division, mate choice strategies, cooperative breeding systems, and
differences in life history traits. In addition, I will discuss how determining survival
rates and quantifying habitat use during the postfledging period is needed for
avian conservation. I will then introduce my study species, Turdus assimilis, and

explain why it was chosen for my research.

POSTFLEDGING PERIOD

The postfledging period has been defined as the period when young birds
are dependent on parental care after fledging the nest (Vega Rivera et al. 2000,
Kopachena and Falls 1993), the period of time after first year birds become
independent from parents but before initiating migration (Morton 1991, Baker
1993, Vega River et al. 1998), or not cleariy defined (Krementz et al. 1989,
Derochers and Harmon 1997). Here I refer to the postfledging period as the time
after young birds have left the nest but before they become independent.
However, the challenges in determining when young birds become completely
independent from their parents makes the period difficult to define. In most
cases, the postfledging period is considered over when adults cease to feed
young (Vander Wall and Hutchins 1983, Moreno 1984, Wolf et al. 1988,
McGowan and Woolfenden 1990, Langen 2000). In cases where it is not
possible to observe feeding, fledglings have been considered independent once
they leave their natal territories (Nilsson and Smith 1985, Sullivan 1989, Evans
Ogden and Stutchbury 1997, Anders et al. 1998, Vega Rivera et al. 2000).
Because parents may provide other forms of care, such as defense from
predators or guidance to foraging areas, time until nutritional independence or
until dispersal from the natal territory may be conservative estimates of the length

of the period.

The difficulty in locating and monitoring fledglings once they leave the nest
is the reason most often cited to explain the lack of information about the
postfledging period (Weatherhead and McRae 1990, Evans Ogden and
Stutchbury 1997, Vega Rivera et al. 2000, Yackel Adams et al. 2001). Young
birds with poor flight skills often initially remain very still, making it very difficult to
find fledglings once they are out of the nest. In addition, it is also often difficult to
observe fledglings without altering their behavior because of strong parental
reactions (Evans Ogden and Stutchbury 1997). While most breeding studies
concentrate on the nesting period alone, the postfledging period is often longer
than, or as long as, the nesting period (see review in Langen 2000). However,
measurements of the length of the period are often based on small sample sizes.
Improving radio-tracking technology, with increasingly smaller and lighter radio-
transmitters, is making it easier to study this difficult period of the life cycle.

The postfledging period may require greater parental investment than the
nesting period. However, there have been few comparisons of provisioning rates
during the nesting and postfledging periods. However, of the few species
studied, feeding rates increased once fledglings left the nest: Eastern Kingbirds
(Tyrannus tyrannus), 50% increase (Morehouse and Brewer 1968); Song
Sparrows (Melospr‘za melodia), 44% increase (Smith 1978); Western Bluebirds
(Sialia mexicana), 60% increase (With and Balda 1990); and White-throated
Sparrows (Zonotn’chia albicollis), 81% increase (Kopachena and Falls 1993).
Feeding rates for both Hooded Warblers (Wilsonia citn'na, Evans Ogden and

Stutchbury 1997) and Northern Wheatears (Oenanthe oenanthe, Moreno 1984)

also increased but the percent increase was not stated. Kopachena and Falls
(1993) also measured the amount of food per trip and found that while feeding
trips per hour increased 81%, the actual amount of food delivered increased only
51%, implying less food is delivered per trip compared to the nesting period and
feeding rates alone may not be the most accurate measure of parental
provisioning. Hence, the postfledging period, in terms of length and provisioning
rates, is as important to reproduction as the nesting period. Most breeding
studies have only contained information about the nesting period (Anders et al.
1998, Yackel Adams et al 2001) but, without including information on the
postfledging period, we cannot fully understand the levels of parental investment,
evolution of mating systems, differences in life history strategies, breeding habitat
requirements, or population productivity, as discussed below.

The majority of postfledging period studies have been primarily concerned
with brood division, a parental care strategy that divides the provisioning care
between the parents, and male parental care. Division of brood care between
the parents after fledging is a parental strategy found in many species (Smith
1978, Moreno 1984, Harper 1985, McLaughlin and Montgomerie 1985,
Kopachena and Falls 1991, Anthonisen et al. 1997, Evans Ogden and
Stutchbury 1997, Vega Rivera et al. 2000, Yackel Adams et al. 2001). The two
primary hypotheses to explain the benefits of this parental care strategy are:
increased foraging efficiency of parents with divided broods and reduction in
predation by separation of the offspring (Smith 1978, Moreno 1984, McLaughlin

and Montgomerie 1985, Anthonisen et al. 1997). Both hypotheses assume a

spatial separation of young, which has been found for many of the species
studied that exhibit brood division (Moreno 1984, Kopachena and Falls 1991,
Anthonisen et al. 1997). In contrast, Western Bluebirds, which do not divide their
broods, do not spatially separate their broods (With and Balda 1990). Brood
division rarely occurs in the nest (Weatherhead and McRae 1990, Anthonisen et
al. 1997) further supporting an association between spatial separation of the
young and brood division. While this evidence weakly supports the hypotheses
that brood division may be a strategy to increase parental foraging efficiency and/
or reduce juvenile predation, there may be other factors influencing whether or
not broods are divided, such as subsequent nesting attempts. For example,
several species do not divide all of their broods produced during a single season,
indicating that there may also be benefits to keeping a brood together during a
season (Harper 1985, Edwards 1985, Vega Rivera et al. 2000). While brood
division has been well documented, its proximate causes remain poorly
understood.

Quantification of each parent's effort or investment during breeding is
essential for understanding why a particular mate is chosen (Evans Ogden and
Stutchbury 1997). For almost all of the few passerine species studied, where
male and female provisioning rates have been compared during the nesting and
postfledging periods, rates were similar between the sexes during the nesting
period and were either not different during the postfledging period, or were higher
for males (Smith 1978, With and Balda 1990, Evans Ogden and Stutchbury

1997, but see Kopachena and Falls 1991). In addition, both Dark-eyed Juncos

(Junco hyemalis, Wolf et al. 1988) and Seaside Sparrows (Ammodramus
maritimus, Greenlaw and Post 1985) reared fewer young through the
postfledging period when males were experimentally removed. Therefore,
females may face selective pressures to choose a mate that contributes
significantly to offspring care during the postfledging period. More information
about the period of dependence after leaving the nest is necessary to understand
mate choice strategies (Omland and Sherry 1994).

The evolution of cooperative breeding in birds may also be better
understood with more information about postfledging care (McGowen and
Woolfenden 1990). Langen (2000) found cooperative breeders had prolonged
offspring dependence compared to noncooperatively breeding species when he
controlled for phylogenetic relatedness, body size, breeding latitude, breeding
habitat, and diet. He hypothesized that prolonged care, with the contributions of
helpers, could increase survivorship of young. Therefore, having helpers may be
most beneficial during the postfledging period. Since investment and feeding
rates may be greater during the postfledging period than during nesting, and
because postfledging dependence may also be longer than nesting (see
references above), the selective pressures driving the evolution of mating
systems may be acting during the dependent period after leaving the nest.
However, most studies have quantified parental care only during nesting (Wolf et
al. 1988, Evans Ogden and Stutchbury 1997). Our understanding of mating
systems is incomplete, at best, without more information on investment and

length of care during the postfledging period.

Differences in life history traits between northern temperate and southern
temperate! tropical bird species have long been recognized and much work has
gone into attempting to understand the selective pressures leading to the
evolution of these differences. However, a paucity of information about
demographic parameters for tropical species makes it difficult, if not impossible,
to understand whether there tmly are differences in life history traits between
latitudes and, if so, why they have evolved (Martin 1996). Survivorship during
and duration of the postfledging period are often cited as missing information
available for few temperate species and almost no tropical species (Karr et al.
1990, Martin 1996, Geffen and Yom-Tov 2000).

The most widely accepted life history trait difference between latitudes is
the consistently smaller clutch sizes of southern than northern breeding species
(Geffen and Yom-Tov 2000). Westmoorland and Best (1987) found that
Mourning Dove (Zenaida macroura), clutch sizes were not limited by care during
the nesting period and hypothesized that, since this species with consistently
small clutch sizes could produce more young through the nesting period, low
postfledging survival could be the factor driving evolution of clutch size. This
finding indicates that while parents may be able to rear more offspring to
fledging, they may be limited by the number of young they can care for during the
dependent period after leaving the nest. Without more information about
fledgling survivorship during care, especially for tropical species, we cannot fully
understand the evolution of avian life history strategies, including optimal clutch

sizes.

Assessing the habitat requirements of species that may be negatively
affected by deforestation, habitat fragmentation, and land-use change is
becoming increasingly critical for biodiversity conservation. Most assessments of
habitat requirements for Neotropical migrants are based on habitats needed for
successful nesting (Martin 1992, Donovan et al. 1995, Robinson et al. 1995) and
survival in wintering and migration stopover habitats (Conway et al. 1995, Moore
and Simons 1995, Petit et al. 1995, Sherry and Holmes 1996). However, several
North American species have been found using habitat types during the
postfledging period, after independence from parents but before migration,
different than those used during nesting (Morton 1991, Bocetti 1993, Anders et
al. 1998, Vega Rivera et al. 1998). This indicates that assessments of habitat
requirements are incomplete without consideration of the postfledging period.
Determining which habitat types juvenile birds survive better in is particularly
important because the postfledging period is considered a time of high mortality
(Baker 1993) potentially limiting population productivity (Drent and Dahn 1980).
With birds using increasingly human altered landscapes, we need to be able to
identify high quality habitats that can sustain viable populations to manage for
their conservation. This is particularly true in tropical areas with high levels of
land-use change and numerous species about which we know very little.

Breeding productivity models are used to determine if populations are
replacing themselves, i.e. are they sources or sinks? The status of the
populations are then used to evaluate quality of the habitats they use and set

standards for conservation through habitat management (Anders et al. 1997).

However, most models of Neotropical migrant population productivity use a value
for juvenile survivorship that is not empirically supported and assumes the
population is producing enough young to replace itself (Sullivan 1989, Anders et
al. 1997). Use of these unsubstantiated values forjuvenile survivorship may lead
to inaccurate conclusions about population viability. Empirically determined l
values for juvenile survivorship are essential for inclusion in models of population ‘-
productivity that can then be applied to assess habitat quality used for

management. Although critical for avian population dynamics modeling and

 

conservation assessments, there exist few known juvenile survivorship rates for
Neotropical migrant species (but see Anders et al. 1997, Yackel Adams et al.

2001) and none exist for Neotropical resident species.

STUDY SPECIES

The White-throated Robin or White-throated Thrush (Turdus assimilis)
ranges from Northern Mexico through northwestern Ecuador and is generally
considered a forest dwelling species (Ridgely and Gwynne 1989, Stiles and
Skutch 1989, Howell and Webb 1995). In Costa Rica, it has largely disappeared
from a portion of its range, possibly due to deforestation, (Stiles and Skutch
1989) and is patchily distributed throughout its range for reasons that are poorly
understood (Skutch 1960). While some individuals are likely altitudinal migrants
following breeding (Stiles and Skutch 1989), little is known about the seasonal
movements of the species in Costa Rica. At my study site in southern Costa
Rica, adult White-throated Robins are very mobile throughout an agricultural
landscape. They nest and forage in cattle pastures, coffee plantations, and
forests (Lindell and Smith 2003, C. Lindell unpublished data).

With increasing rates of deforestation and land-use change, often to
agricultural systems, especially in the tropics, it is important for us to understand
how species use these agroecosystems. Different habitat types within a
landscape may be important for certain behaviors or stages of the lifecycle (Law
and Dickman 1998, Graham 2001). Therefore, in order to conserve biodiversity
in increasingly anthropogenic landscapes, we need more information about
survivorship and reproductive rates of species like the White-throated Robin that

can use agricultural habitat types along with forest. The White-throated Robin is

10

especially interesting because it successfully nests and forages in both coffee
plantations and cattle pastures at my study area but has drastically declined in
the upper Central Valley of Costa Rica where the majority of the forest has been
converted to agricultural habitat types, including coffee and pasture. If we are to
predict how land-use change will affect species, we need a better understanding
of how demographic parameters, such as reproductive success and survivorship,
are affected by habitat use at all stages of the life-cycle.

Despite its importance, there is very little information about the
postfledging period for any Neotropical resident species. While there have been
many hypotheses about the duration of and survivorship during the period, most
available data are based on few observations without empirically determined
values (e.g. Snow and Snow 1963, Skutch 1976). Data on survivorship, habitat
use, and movements of postfledging White-throated Robins are important for
understanding the relative quality of agricultural habitat types and will be some of

the first reported for a Neotropical resident bird.

I]

CHAPTER 2
SURVIVAL, HABITAT USE, AND MOVEMENTS OF FLEDGLING
WHITE-THROATED ROBINS IN TROPICAL AGRICULTURAL HABITATS

ABSTRACT

I used radio-telemetry to study the behavior of White-throated Robins
(Turdus assimilis) during the postfledging dependent period. The study was
conducted in a mixed agricultural and forested landscape in southern Costa Rica
from March through August of 2001 and 2002. A transmitter was attached to one
fledgling per brood (n = 53). Each bird was located daily until dispersal from their
natal areas. I compared survivorship, habitat use, and movements for fledglings
from nests in coffee plantations and cattle pastures. Birds that fledged from
nests in pasture were more likely to survive until dispersal from the natal area
than were birds from nests in coffee. The probability of surviving the first three
weeks out of the nest was 0.67 :I: 0.07 (SE) for fledglings from nests in all
habitats, 0.58 i 0.10 for fledglings from nests in coffee, and 0.74 :t 0.26 for
fledglings from nests in pasture. Fledglings from nests in pasture left the natal
habitat type at younger ages than did birds from nests in coffee and most birds
from both habitats moved into forest when they left their natal habitat. Pasture
habitat was rarely used during the postfledging period while coffee was used
extensively. Fledglings that remained in agricultural habitats (pasture or coffee)
were less likely to survive until dispersal than were those that moved into

forested areas. Average daily distances from the nest gradually increased until

12

fledglings dispersed away from the natal area, always into forest, and were not
different for birds from pasture or coffee. While White-throated Robins can nest
successfully in agricultural habitats, the use of forest positively influenced

survivorship of young during the postfledging dependent period.

13

INTRODUCTION

Neotropical forests are rapidly being converted to agricultural land covers
(Hartshom 1992, Houghton 1994), increasing our need for information about the
species using these agroecosystems (Pimental et al. 1992, Perfecto et al. 1996,
Vanderrneer and Perfecto 1997). Cattle pastures and coffee plantations are two
of the most prominent agricultural habitats in the Neotropics (Feamside 1993,
Rice and Ward 1996). In recent years, a number of studies have examined bird
use of Neotropical coffee plantations (e.g. Greenberg et al. 1997a, Greenberg et
al. 1997b, Wunderle 1999, Johnson 2000, Jones et al. 2002) and cattle pastures
(Greenberg and Ortiz 1994, Cardoso da Silva et al. 1996, Estrada et al. 1997)
although demographic information for species using tropical agricultural habitats
is extremely limited (but see Lindell and Smith 2003). The quality of Neotropical
coffee plantations and cattle pastures, in terms of their conservation value and
the long term viability of the avian populations using them, cannot be determined
without measures of demographic parameters such as survivorship or
reproductive success (Garshelis 2000).

The postfledging period is considered one of the least understood phases
of the avian life-cycle (Sullivan 1989, Weatherhead and McRae 1990, Rappole
1995) largely because of the difficulty in observing young birds after they leave
the nest (Moreno 1984, Evans Ogden and Stutchbury 1997). lnforrnation on the

postfledging period for tropical birds is scarce and considered one of the missing

14

links in our understanding of avian life-history patterns (Karr et al. 1990, Martin
1996, Geffen and Yom-Tov 2000). The postfledging dependent period may be
as long as or longer than the nesting period (Smith 1978, With and Balda 1990,
Langen 2000) and survivorship of juvenile birds through the entire period of
dependence is a more accurate assessment of reproductive output than is
nesting data alone (Anders et al. 1997). Despite its importance in understanding
population viability and dynamics, anecdotal information about the postfledging
period is available for only a few Neotropical resident songbird species (e. 9.
Snow and Snow 1963, Skutch 1976).

The White-throated Robin (Turdus assimilis) is a Neotropical resident with
a range from Mexico through Ecuador (Skutch 1960, l-lilty and Brown 1986,
Ridgley and Gwynne 1989, Stiles and Skutch 1989, Howell and Webb 1995). Its
breeding range is patchily distributed in Costa Rica for reasons that are poorly
understood (Skutch 1960). The species has typically been considered a forest
species (Ridgley and Gwynne 1989, Stiles and Skutch 1989, Howell and Webb
1995) and in recent decades it has nearly disappeared from a portion of its range
in the upper Central Valley of Costa Rica (Stiles and Skutch 1989) that has
undergone extensive urbanization and deforestation to pasture and coffee
(Sénchez—Azofeifa 1996). However, White-throated Robins are still common at
my study site in the Coto Brus area of southern Costa Rica, where they nest in
both coffee plantations and cattle pastures. The two regions of the country differ
in that large tracts of primary forest remain in Coto Brus but are no longer

present in the upper Central Valley (Stiles and Skutch 1989).

15

Within a landscape, a species may require several habitat types for
different behaviors (Graham 2001) or stages of the lifecycle (Law and Dickman
1998). Postfledging birds may use different habitat types than those used by
their parents during nesting (Bocetti 1993, Anders et al.1998, Ganey et al. 1998,
Vega Rivera et al. 1998). Hence, habitat requirements for successful production
of young cannot be assessed solely from nest locations. Therefore, while White-
throated Robins can nest in agricultural habitats, forested areas may be
important for other behaviors or stages of their life cycle, including the
postfledging dependent period.

Previous work at our study site showed comparable daily mortality rates
for above ground cup nests, of all species, in pasture, coffee and forest habitats
(Lindell and Smith 2003). However, species-specific studies that include
postfledging survivorship in measures of reproductive success are necessary to
establish the value of these different habitat types to particular populations. In
addition, young birds fledged from nests in pasture and coffee may use, or
survive better in, forest or other habitat types that differ from their parents'

nesting habitat types.

16

OBJECTIVES

My objectives in this study were to document the survivorship, habitat use,
and movements of postfledging White-throated Robins and to compare these

variables for birds fledged from nests in coffee plantations and in cattle pastures.

l7

METHODS

Study Site

I conducted the study at Las Alturas, a large farm in the Coto Bms region
of Costa Rica (8° 57' N 82° 50' W, Figure 1), March through August of 2001 and
2002. Prior to 1952, Coto Brus, an area of approximately 90,000 ha, was almost
entirely forested. However, by 1990, coffee and cattle production were common
on forrnerly forested land (Manger 1992).

Las Alturas is on the Pacific slope of the Talamanca Mountain range at an
elevation of 1300 to 1500 meters. The farm is an active cattle ranch comprised
of cattle pastures, abandoned coffee plantations, and forest fragments and is
surrounded by seasonal montane wet forest (Holdridge et al. 1971 ). Both the
pasture and coffee areas are used for cattle grazing. The pastures are
characterized by non-native grasses with few shrubs or mid-story vegetation,
little cover, and scattered tall remnant trees. The coffee plants are 1-3 m high
with varied densities of 5-7 m high shade trees. The coffee plantations were
abandoned in 1995 and have not been harvested or treated with agrochemicals
since. However, the overstory trees are usually trimmed twice a year. The
agricultural portion of the farm is surrounded by forest, some of which has been
selectively logged and some of which is primary forest in the Las Tablas
Protected Area, part of the international 600,000 ha La Amistad Biosphere

Reserve.

18

Nest Searching and Radio Attachment

White-throated Robins in our study area nest from March through August
with a peak in May. I began nest searching in March of both years, concentrating
in one pasture area and one coffee area in 2001 and expanding to two areas of
each habitat type in 2002. However, I monitored nests found on an ad-hoc basis
throughout the farm (Figure 1). Once located, nest contents were checked every
2 to 4 days.

All nestlings in each nest were marked with a metal numbered band and a
unique combination of color bands approximately two days before fledging.
Measurements were taken of the mass and the lengths of the wing chord, tail,
bill, and tarsus at this time as well. I was not able to determine the sexes of
nestlings or fledglings. To insure independence of samples, transmitters were
placed on only one nestling per nest (brood size ranged from one to three
nestlings). In four instances transmitters were placed just after fledging, either
because the nest was too high to monitor or the nestling fledged early without a
transmitter. For consistency, transmitters were always placed on the nestling
with the greatest mass. In two cases, when the transmitters were placed after
the birds fledged, l was not able to determine if I placed the transmitter on the
heaviest sibling.

The transmitters were attached with thin elastic using the leg-hamess
method (Rappole and Tipton 1991 ). Powell et al. (1998) found this method of
radio attachment did not have negative effects on migratory Wood Thrush

(Hylocichla mustelina) when radio-tagged birds were compared to banded-only

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20

birds. The transmitters weighed less than 3.5% of the average body mass for
nestlings, at the age of placement, and less than 3% of the average adult body
mass. In 2001, I used both model BD-2A and BD-2G transmitters from Holohil
Systems Ltd. Because I did not observe differences in behavior or mortality rates
between birds tagged with the two models, I used only the latter in 2002, which

have a longer battery life (10 versus 3 weeks) but are heavier (1 .79 versus 0.99).

Data Collection

Once out of the nest, fledglings were located daily, using a hand-held five-
element Yaggi antennae and a Wildlife Materials TRX-20008 receiver, until the
bird died, dispersed to inaccessible areas, or the transmitter battery died. Each
time a radio-tagged bird was sighted, I recorded habitat type along with flight
distances, estimated to the nearest meter, and perching height above the
ground, estimated to the nearest half meter. Habitat types were recorded for
each location as pasture, coffee, forest, or other (e.g. road). The same data
were recorded for banded-only siblings of radio-tagged birds when possible.
When signals were lost, I checked from high points around the study area for
several consecutive days and, on three occasions when the birds were too young
to have entirely left the study area (based on prior observations of flight ability), I
considered them to be dead. I continued to search for lost signals throughout the
season.

The detection range for transmitters varied with terrain but, from high

elevation points, was up to 3 km. I used a global positioning system (GPS,

21

GeoExplorer ll, Trimble Navigation, Ltd.) to record the location where the bird
was first detected each day. To increase accuracy, I set filters in the GPS unit
and averaged a minimum of 120 positions for each point. The GPS points were
imported into a Geographic lnfon'nation System (GIS, ARC/GIS° software, ESRI,
Inc.) in a Universal Transverse Mercator projection to measure movement
distances. l estimated the distances from each nest to the closest forested area

using a combination of measurements on the ground and an aerial photograph.

Statistical Analyses

Except where stated otherwise, ages are reported in days posthatching
with hatching day as day one. I used SAS® (SAS/STAT Institute Inc.) and
STASTICATM (StatSoft, Inc.) for analyses except for the G-tests of independence,
which were calculated with a William's correction according to Sokal and Rohlf
(1995). While I tracked 53 fledglings overall, the sample sizes vary due to
mortality and transmitter battery failure. All two-way tests are two-tailed and
means are reported as :t standard deviation, except where stated otherwise.

Survival probability curves were calculated using a Kaplan-Meier
procedure (Allison 1995, Winterstein et al. 2001). I tested for differences in
survival probability curves between birds fledged from nests in pasture versus
coffee with the Wilcoxon statistic. I chose the Wilcoxon test because it is
weighted towards early occurring events (Allison 1995, Winterstein et al. 2001)
and the majority of the mortalities occurred soon after fledging. There was no

significant difference between 2001 and 2002 survival probabilities (n = 17, 36,

22

Wilcoxon, P >0.1), so I combined data for the 2001 and 2002 field seasons. To
determine if there was a relationship between survivorship and habitat use, I
used the Wilcoxon test to compare survival probability curves of fledglings that
moved into forested areas to those that remained within agricultural habitats
(coffee or pasture).

For each bird that survived until dispersal from the natal area, I
determined the number of daily locations prior to dispersal that were in each
habitat type. To compare percentages of points located in forest for birds from
nests in coffee and pasture, that survived and were tracked until dispersal,
Wilcoxon two-sample tests were used. I also determined the age that each bird
moved out of its natal habitat and the habitat type that he/she moved into first.
To test for a relationship between the natal habitat type and the number of days
after fledging that birds left their natal habitat type, I used a G-test of
independence with the William’s correction. I used the same test to determine if
there was a relationship between the habitat type of the nest from which birds
fledged and whether they remained in agricultural habitats or moved into forest
prior to mortality, dispersal from the natal area, or censoring (unknown fate).

Daily distances from the nest were calculated for successive ages using
the Animal Movement Extension (Hooge and Eichenlaub 1997) to ArcView"
(ESRI, lnc.). During my 2001 field season, I observed fledglings remaining
relatively close to the nest and then making a one-directional movement, of
greater than 75 m, away from their natal area. I termed this the initial dispersal

movement and was rarely able to track birds after this movement. I recorded the

23

age on the day of dispersal and the minimum distance each fledgling moved the
first day of the dispersal movement. When I was not able to get accurate GPS
positions for the first location after dispersing, I used an aerial photograph to
determine a minimum distance moved. For paired comparisons of distances
from the nest, at successive ages, between fledglings from nests in coffee and
pasture, I used the Wilcoxon signed-ranks test. I compared age at dispersal for
fledglings from each natal habitat type with a Wilcoxon two-sample test.

To determine if radio-tagged birds behaved differently than non-tagged
birds, I compared perching heights and flight distances of banded-only birds and
their radio-tagged siblings, observed on the same day, with Wilcoxon signed-
ranks tests. To determine if coffee or pasture nests were closer to forested areas
or if there were differences in age at fledging, brood size, nestling mass on the
day of radio placement, or dispersal age for birds fledged from nests in pasture

and coffee, I used Wilcoxon two-sample tests.

24

RESULTS

Survival

l tracked 17 fledglings in 2001 (10 from nests in coffee, 4 from pasture,
and 3 from other habitat types) and 36 in 2002 (14 from nests in coffee, 21 from
pasture, and 1 from another habitat type). The cumulative probabilities of
surviving through the first, second, and third weeks out of the nest were 0.76 i
0.06, 0.69 i 0.07, and 0.67 i 0.07(SE, n = 53) respectively, for all birds tracked;
0.88 i 0.07, 0.79 i 0.08, and 0.74 i 0.26 (SE, n =25) respectively, for birds from
nests in pasture; and 0.63 i 0.10, 0.58 i 0.10, and 0.58 :l: 0.10 (SE, n =24)
respectively, for birds from nests in coffee (Table 1).

Survivorship of fledglings from nests in coffee was lower than for
fledglings from nests in pasture (n = 24, 25, Wilcoxon, P < 0.05, Figure 2). Birds
that remained in agricultural habitats (pasture and coffee) during the first three
weeks out of the nest, as opposed to moving into forest, were more likely to die
(n = 12, 38, Wilcoxon, P < 0.0001, Table 2, Figure 3). Because fledglings that
moved into forest were less likely to die than were those that remained in
agricultural habitats, and the nests in pasture were significantly closer to forest
(2 = 45.6 :t 47.9 m, n = 24) than were the nests in coffee, ()7 = 83.9 i 48.0 m, n
= 24, Wilcoxon, P < 0.01 ), I performed a post-hoc log-linear analysis to determine

if there was a three-way interaction between

25

Table 1. Kaplan-Meier survival rates during the first three weeks out of the nest
for fledglings from nests in pasture and coffee habitats. Survival rates are
calculated only on days when mortalities were detected.

Day Survival

(Postfledging) rate
Fledglings from Nests in Pasture

4 0.96
5 0.92
6 0.88
7 a 0.88
8 0.84
9 a 0.84
11 a 0.84
13 0.79
14 0.74

Fledglings from Nests in Coffee

0 0.83
1 0.71
3 0.67
4 0.63
8 a 0.63
9 0.58
10 a 0.58
11 ‘ 0.58
13 a 0.58
14 a 0.58
18 a 0.58

Survival

SE

0.04
0.05
0.07
0.07
0.07
0.07
0.07
0.08
0.09

0.08
0.09
0.10
0.10
0.10
0.10
0.10
0.10
0.10
0.10
0.10

Total

Mortalities

ODU'I-h-h-hwwN-e

Total
Rm

24
23
22
21
20
19
18
17
16

20
17
16
15
14
13
12
11
10

 

a censored observation (unknown fate)

b The last time these birds were located, the day before they died, one of them
was in pasture, four were in forest, and one was in coffee.
°The last time these birds were located, the day before they died, seven of them
were in coffee, two were in forest, and one was in another habitat type on the

farm (not coffee, pasture, or forest).

26

 

 

+ from pasture nests

—I- from coffee nests

 

 

 

.0 .0
09 ‘9

.0
N

 

 

 

Cumulative survival probability
.0
0?. .

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

.0
‘i‘

 

 

.0
A

 

012 3 4 5 6 7 8 9101112'1314151617181920
Days postfledging

Fig. 2. Comparison of survival probability curves for fledglings from nests in
coffee versus pasture during the first three weeks out of the nest. Daily survival
probabilities are cumulative (vertical bars represent SE) and are calculated using
a Kaplan-Meier procedure.

27

Table 2. Kaplan-Meier survival rates during the first three weeks out of the nest
for birds that remained in agricultural habitat (coffee or pasture) and birds that
moved into forest habitat. Survival rates are calculated only on days when

Total
Mortalities

 

 

mortalities were detected.

Day Survival Survival
(Postfledging) @te SE

Fledglings that remained in Agricultural Habitat
0 0.67 0.14 4
1 0.50 0.14 6
6 0.42 0.14 7
9 a 0.33 0.14 8
10 a 0.33 0.14 8
14 0.17 0.14 9
Fledglings that moved into Forest Habitat

1 0.97 0.03 1
3 0.95 0.04 2
4 0.92 0.05 3
5 0.89 0.05 4
7 a 0.89 0.05 4
8 a 0.86 0.06 5
10 a 0.86 0.06 5
1 1 a 0.86 0.06 5
13 0.83 0.06 6
14 a 0.83 0.06 6
18 " 0.83 0.06 6
19 a 0.83 0.06 6

Total
RemML

ANOJU'IOJG

36
35

33
32
30
29
28
25
24
23
22

 

a censored observation (unknown fate)

28

 

 

 

 

 

12
g :
'5 .
g 0.81
a I + moved into forest
.> 0.6“ . .
g « + stayed In agricultural
(I)
g 0.4—
g .
a .
g .
O 0.2“.

I
O I I

 

 

 

012 3 4 5 6 7 8 91011121314151617181920

Days postfledging

Fig. 3. Survival probability curves for fledglings that remained within agricultural
habitats and fledglings that moved into forest habitat during the first three weeks
out of the nest. Daily survival probabilities are cumulative (vertical bars represent
SE) and are calculated using a Kaplan-Meier procedure.

29

natal habitat type, distance of the nest to forest, and survivorship through the first
week out of the nest (when the majority of the mortalities occurred). Natal habitat
type, distance of the nest to forest and survivorship were independent (X2 = 0.16,
df = 1, n = 49, P > 0.6). In addition, there was no two-way relationship between
distance of the nest to forest and survival when we controlled for natal habitat
type (X2 =0.64, df = 1, n = 49, P > 0.4). Reflecting the significant difference in
survivorship curves between fledglings from nests in coffee and pasture, the
relationship between natal habitat type and survivorship was marginally
significant when we controlled for distances of the nests to forest (X2 =3.22, df =
1, n = 49, P < 0.07).

Birds from nests in coffee and pasture did not differ in the ages at which
they fledged ()7 = 13.8 i 1.7 days, n = 24, birds from coffee; 32 = 13.9 :I: 1.6 days,
n = 24, birds from pasture, Wilcoxon, P > 0.9). They also did not differ in their
nestling masses on the day of transmitter placement ()7 = 48.7 :I: 3.3 g, n = 23,
birds from coffee; )7 = 48.4 :I: 3.2 g, n = 24, birds from pasture, Wilcoxon, P >
0.6), or in their brood sizes (x = 2.0 i 0.7, n = 24, birds from coffee; i = 2.0 i
0.7, n = 25, birds from pasture, Wilcoxon, P > 0.9).

I observed several species of avian predators including Chestnut-
mandibled Toucans (Ramphastos swainsonii), Yellow-headed Caracaras
(Milvago chimachima), Roadside Hawks (Buteo brachyurus), and Emerald
Toucanets (Aulacorhynchus prasinus) perched in the coffee plantation overstory
trees and scanning the evenly planted coffee plants below them. I observed one

predation event on a fledgling by an American Swallow-tailed Kite (Elanoides

30

forficatus) and was able to recover the majority of the transmitters after
mortalities occurred. While I recovered a few transmitters with tooth marks, I
rarely saw mammals. Snakes were observed eating nestlings, including one with
a transmitter, but were not observed depredating fledglings. All recovered
transmitters were either attached to a depredated bird or had marks on them

suggestive of predation.

Habitat Use

The percentage of points located in forest prior to dispersal (for the birds
that survived and were tracked until dispersal) was lower for fledglings from nests
in coffee (17.5 i 25.9 %, n =10) than for fledglings from nests in pasture (64.0 :I:
38.0 %, n = 15, Wilcoxon, P < 0.01). Of the fledglings I tracked until dispersal,
six of the 10 from coffee nests were always located in coffee prior to dispersal.
The other four were found in forest and coffee, and none were ever located in
pasture. Of the 15 birds from pasture nests tracked until dispersal, four were
found only in forest and five were located in pasture and forest. One was located
only in pasture and the remaining fledglings from pasture were located in various
combinations of habitat types (Table 3).

Fledglings from nests in pasture moved from their natal habitat into
another habitat type more quickly than did birds from nests in coffee (2: 2.90 :t
0.36 days postfledging, for birds from pasture; 2 = 9.87 i 6.47 days postfledging,

for birds from coffee, n = 24, 15, Wilcoxon, P < 0.01). Fledglings moved to forest

3]

Table 3. Locations in each habitat type, prior to dispersal, for fiedglings from
nests in coffee and pasture that survived and were tracked until dispersal from
their natal areas.

Locations Er habitat mg Habitat type Days after
moved to fledging left

Bird ID Nest typea Forest ("/o)b Pasture Coffee after natala natal habitat

 

D6-01 c 8 (80) o 2 F 3
06-02 c 7 (47) o 8 F 9
M2-02 c 6 (23) o 20 F 19
N4-01 c 2 (25) o 8 F 6
A3-01 c o (0) o 16 F 18
14.02 c o (0) o 13 F 14
K3-02 c o (0) o 18 F 21
03-01 c o (0) o 12 F 13
U4-02 c o (0) o 12 F 13
U5-01 c o (0) o 9 F 10
H3-02 P 10 (100) o o F 1
.1-02 P 28 (100) o o F 1
s-oz P 24 (100) o o F 1
V5-02 P 6 (100) o o F 1
W292 P 17 (90) 2 o F 2
L5-02 P 16 (89) 2 o F 3
A3-02 P 20 (87) 3 o F 3
MU-02 P 5 (36) 9 o F 7
R-02 P 11 (34) 21 o F 6
F3-01 P 8 (80) 1 1 F 2
M5—02 P 12 (80) 1 2 c 2
153-02 P 13 (76) 2 2 F 3
H-01 P o (0) 8 14 c 2
K5-01 P o (0) o 4 c 1
T-02 P o (0) 18 o F 19

 

a P = pasture, C = coffee, F= forest
b Percentage of total locations recorded in forest

32

74% (n = 43) of the time when they first left their natal habitat type. Fledglings
from nests in coffee more often remained in coffee (6 = 10.49, df = 2, n = 48, P <
0.01), as opposed to moving into forest, while birds from pasture nests more
often moved into forest.

Fledglings from pasture nests moved to forest earlier (6 = 10.53, df = 2, n
= 41, P < 0.01) than did fledglings from coffee nests. Since pasture nests were
closer to forest and birds from pasture nests moved into forest more quickly, I ran
a post-hoc log-linear analysis to determine if there was a three-way interaction
between natal habitat type, distance of the nest to forest, and the age fledglings
moved into forest. We did not find a significant three-way interaction (X2 = 0.31,
df = 1, n = 37, P > 0.5). However, birds from nests closer to forest moved into
forest at younger ages when natal habitat type was controlled for (X2 = 12.08, df
= 1, n = 37, P < 0.001) and birds from nests in pasture also moved into forest at
younger ages than did birds from coffee, when the distance of nests from forest
was controlled for (X’- = 5.64, df = 1, n = 37, P < 0.05). All fiedglings that
survived dispersed into forested habitat, moving gradually further from the natal

area and were rarely located again within the agricultural portion of the farm.

Movements
Average daily distances from the nest were not different for fledglings from
nests in pasture and coffee (n = 16,Wilcoxon signed-rank, P > 0.2) and increased

from an average of 11.2 i 11.0 m (n = 17) on day 14 posthatching to 133.9 i

33

450
400 i

i (21)
350 .

 

(1_8)

 

 

m
0
O

 

: ‘23:)
250

200 3

‘ (20) (so) (23’ T

 

(25)

 

Distance (m)

 

 

 

 

 

 

     
 
 

‘ (30
< (10) (17) (21) (30) (33) (313;) T

 

 

13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

Age (days posthatching)

Fig. 4. Average daily distances from nests at successive ages prior to dispersal.
Bars represent SD and sample sizes are in parentheses.

34

269.4 m (n = 18) on day 29 posthatching (Figure 4). The age at dispersal ranged
from 23 to 46 days posthatching, and was not significantly different for birds from
nests in coffee and pasture ()7 = 30.0 :t 5.3 days, for birds from nests in coffee; >7
= 32.6 :I: 7.1, for birds from nests in pasture, n = 10, 15, Wilcoxon, P > 0.3).
Fledglings moved 90 to 500 m on the first day they dispersed from the natal area.
The minimum average first day dispersal distance was 297.5 1 157.0 m (n = 21,
Figure 5). I observed a seasonal trend in age at dispersal from the natal area
and ran a post-hoc analysis, using Spearrnan rank-order correlation, to
determine if there was a relationship between fledging date and age at dispersal.
I found a significant negative relationship between fledging date and age at
dispersal (rs = -0.51, n = 28, P < 0.01, Figure 6).

The masses of the radio-tagged nestlings were significantly greater than
the average masses of their siblings (n = 38, Wilcoxon signed-rank, P <0.001)
but average wing chord, tail, bill, and tarsus lengths were not different (n = 37,
36, 37, and 38, respectively, Wilcoxon signed-rank, P >01 in all cases),
indicating similar developmental stages for radio-tagged and sibling nestlings.
Because of the difficulty in detecting color-banded siblings of radio-tagged
fledglings, we were not able to consistently observe them. However, color-
banded fledglings and their radio-tagged siblings, when observed on the same
day, were perched at the same heights above the ground (n = 33, Wilcoxon
signed-rank, P > 0.7) and flew the same distances when approached (n = 31,
Wilcoxon signed-rank, P > 0.7). Of the birds tagged as nestlings in 2001, three

were resighted in 2002 with their transmitters. One of these was attending an

35

active nest and another was recaptured and the transmitter was removed. The
recaptured second year bird had full adult plumage and I did not observe any

skin irritation from the harness.

36

.559. 8898 SE 9638 EcEc>oE 32636 9.6. m @5me :65 32 :5 9mm: :85 Leo: mcEfiEe .0 E833
EmEo>oE .833 85 95882: .323 no... new 30:63 8&3 .3568: 05 Co £ccEc>oE cEmem .m .9”.
z

8362 8.2 82 8n 6 68 82 8832.. 86:6 LE
25 8:8.qu 8832 8.:

 

37

 

338

Age (days posthatchingj
w
01

 

 

 

 

30 _

25 ~

20 T f I T 7 T T T I

4-Apr 14-Apr 24-Apr 4-May 14-May 24-May 3-Jun 13-Jun 23-Jun 3-Jul 13-Jul
Pledging date

Fig. 6. Correlation between fiedging date and age at dispersal from the natal area
(r. = -o.51, n = 28, P < 0.01).

38

DISCUSSION

Survival

My results indicate that young birds from nests in coffee had lower survival
because they spent more time in coffee prior to moving into forest compared to
birds from pasture that moved more quickly into forest. The majority of the
mortalities occurred the first few days out of the nest and more of these occurred
when birds were in coffee than forest or pasture. The higher mortality rates of
coffee-fledged birds compared to pasture-fledged birds may be due to the even
distribution of coffee plants, with little cover between them and the shade trees 2
to 6 m above. This vegetation structure may have made it easier for perched
avian predators to locate young birds that could not yet fly well. In addition,
because the coffee provided some cover, unlike pasture where there was almost
none, the fledglings may have had less motivation to leave coffee, illustrated by
the older ages at which coffee-fledged birds left their natal habitat type compared
to pasture-fledged birds, despite the potential threat from avian predators.

I found a trend similar to the one found by Sullivan (1989) for juvenile
Yellow-eyed Junco (Junco phaenotus) survival during the dependent period. The
majority of mortalities occurred during the first week out of the nest when birds
were not able to fly well, fewer occurred during the second week out, and very

few occurred during the third week, when flying skills were better and while

39

parents were still feeding them. Like Sullivan, I also found predation, as opposed
to starvation, to be the primary cause of death during the dependent period.

Fledglings from nests in pasture and coffee came from clutches of the
same sizes, weighed the same as nestlings, fledged at the same ages, and
moved the same distances once they were out of the nest. This indicates that the
difference in survivorship was due to differing predation rates in the habitat types
the fledglings used as opposed to a difference in fledgling quality.

Mammalian predators had the largest impact during nesting for Lark
Buntings (Calamospiza melanocorys), while raptors were responsible for more
mortalities during the postfledging period (Yackel Adams et al. 2001). While
more information is needed about White-throated Robin nesting and fledgling
predators, avian predators may be most important during the postfledging period
when young birds are in open habitats.

Juvenile survival rates are important for understanding why life history
traits differ between tropical and temperate nesting species (Martin 1996, Geffen
and Yom-Tov 2000). For example, optimal clutch sizes may be a function of the
number of young, per breeding attempt, that can successfully be reared until
independence (Westmoorland and Best 1987), illustrating the importance of
juvenile survival during the dependent period after leaving the nest for
understanding clutch size evolution. Survivorship during the first three weeks out
of the nest was higher for White-throated Robins, 0.67 i 0.07 (SE), than has
been reported for most songbird species breeding in North America. For

example, Wood Thrush and Lark Buntings were studied using the same

40

techniques and their probabilities of surviving the first three weeks out of the nest
were 0.43 (Anders et al. 1997) and 0.37 (Yackel Adams et al. 2001),
respectively. Our results indicate that postfledging juvenile survival is higher for
this tropical species but more empirically determined estimates of juvenile
survival are needed for tropical-temperate comparisons.

To ensure independence of sampling units I followed only one nestling
per brood. It is possible that the survival rates were higher than the average for
fledglings in the population because I choose the nestling with the greatest mass
from each brood as the sampling unit. However, since measurements of wing,
bill, tarsus, and tail lengths were not different between the tagged birds and their
siblings, I think all nestlings were similarly developed and my survival estimates

were representative of the population in general.

Habitat Use

Birds from nests in both coffee and pasture most often moved into forest
after leaving their natal habitat type. Fledglings from nests in coffee never
moved to pasture and birds fledged from pasture rarely moved to coffee,
although coffee and pasture habitats are adjacent in many areas. All juvenile
birds dispersed into forest and all but one fledgling moved out of the agricultural
portion of the farm after leaving the natal area, despite the fact that dispersal
distances were often less than the extent of the agricultural land. These results,
along with the finding that birds located in forest, as opposed to coffee, were

more likely to survive, indicate that forest was important during the postfledging

41

dependent period. Previous work on Wood Thrush also showed habitat use
during the postfledging period, prior to migration, differing from during nesting
(Anders et al. 1998, Vega Rivera et al. 1999).

Movements

White-throated Robins moved gradually further from the nest and
eventually dispersed from the natal area, a pattern also reported for juvenile
American Robins (Turdus migratorius, Weatherhead and McRae 1990). Since I
was rarely able to observe individuals without altering behavior and was unable
to locate fledglings consistently after they dispersed, I do not know if juvenile
birds became independent at dispersal. On several occasions, parents were
reusing nests after the fledgling l was tracking had dispersed great distances
from the natal area, indicating fledglings were independent. However, I also
observed a juvenile bird, over 1 km from its natal area, who had been out of the
nest for over two months and had dispersed over a month prior, associated with
an adult bird who approached the juvenile with food and gave an alarm call when
I approached, indicating that this fledgling was not independent. While more work
is needed on the period after dispersal from the natal area, I believe there is
some variability in the length of the dependent period for this species.

The length of the dependent postfledging period may be longer for tropical
species, reflecting greater parental investment in young and resulting in
increased survival, compared to temperate species (Martin 1996, Geffen and

Yom-Tov 2000). The average age at dispersal for White-throated Robins, 31

42

days, was similar to the ages at independence for American Robins, 37 days,
(Weatherhead and McRae 1990) and for Wood Thrush, 32.5 i 0.6 days (Anders
et al. 1998), or 28 to 36 days (Vega Rivera et al. 2000), in North America. Other
North temperate breeding species have comparable ages at independence
(Green-tailed Towhee, Pipilo chlorura, up to 30 days, Morton et al.1991; Song
Sparrow, Melospiza melodia, about 30 days, Hochachka and Smith 1991; Marsh
Tit, Parus palusfn's, 31 to 35 days, Nilsson and Smith 1985; Hooded Warbler,
Wilsonia citrina, 37 to 51 days, Evans Ogden and Stutchbury 1997). A tropical
species, the Cocoa Thrush (Turdus fumigatus), also had similar, postfledging
parental care, 36 to 46 days, in Trinidad (based on observations of three families,
Snow and Snow 1963). We were unable to determine if young birds were always
independent at dispersal, making it difficult to make definite comparisons.

Juvenile birds dispersed at younger ages when they fledged later in the
season. Parents may allow juveniles to remain within natal territories after
nutritional independence, potentially increasing juvenile survival at little cost to
themselves (Martin 1996). Because most parent birds were not color-banded, l
was not always able to follow adults' subsequent nesting attempts. However,
since birds may be more likely to renest earlier in the season and adults may
have remained in their nesting areas longer when they were attending a second
brood, fledglings from first broods may remain in natal areas with their parents,
and disperse at older ages, when their parents are renesting.

While the postfledging period may be longer and require a greater

parental investment than the nesting period, most breeding studies continue to

43

document nesting alone. However, little can be definitively concluded about the
reproductive strategy of a species or the reproductive output of a population
without information about the survivorship and habitat requirements of young
birds. Such information is also necessary to predict the susceptibility of a

population to decline in the face of land use change.

Conservation Implications

Las Alturas is surrounded by pristine and selectively logged forest and
forest fragments are interspersed within the agricultural habitats. Reproductive
output of the White-throated Robin at Las Alturas could be lower for birds nesting
in coffee than in pasture due to decreased survivorship during the postfledging
period, which I have shown is linked to fledgling use of low quality coffee habitat.
The speed with which fledglings from pasture nests left pasture indicates it is not
desirable habitat. White-throated Robins can nest successfully in agricultural
habitat types (Lindell and Smith, 2003). However, my results suggest a threshold
level of forest may be necessary to meet the habitat requirements of fledglings
and, possibly, allow the persistence of local populations. If this is true, it could
explain why the species has disappeared from the upper Central Valley of Costa
Rica, an area without large forest tracts. My findings indicate the importance of
investigating habitat use during all stages of the lifecycle.

My results also suggest that shaded and unshaded monoculture coffee
plantations similar to those at Las Alturas, with little overstory diversity or

structural complexity (see Mogul and Toledo 1999), may serve as ecological

traps in that survival of fledglings is reduced, compared to that of fledglings using
primarily forest. However, information on nesting densities and success rates
across habitat types would provide more definite evidence. No White-throated
Robin nests were found during several seasons of searching in adjacent forest
habitat (C. Lindell unpublished), indicating that nesting densities were much
higher in the agricultural habitats. Skutch (1966) suggested that tropical birds
may nest more densely in human altered habitats, with thinned vegetation, than
in adjacent forest. The increased visibility in more open agricultural habitats, as
opposed to tropical forests, may make it easier for birds sitting on nests to detect
and escape from predators. The vegetation structure of the coffee plantations
may be similar enough to the streamside forest habitat White-throated Robins
presumably nested in historically to be attractive to nesting adults and to provide
little motivation to the fledglings to leave. However, this same structure may
make the habitat unsuitable for fledglings because it provides perches for
predators and little cover for hiding. lnforrnation on postfledging survivorship and
habitat use are essential for assessments of both habitat quality and population
viability, particularly for species in the Neotropics where forests are rapidly being

converted to agroecosystems.

45

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