SHORT COmmUNICATIONS 565
The Condor, Vol. 111, Number 3, pages 565–569. ISSN 0010-5422, electronic ISSN 1938-5422.  2009 by The Cooper Ornithological Society. All rights reserved. Please direct
all requests for permission to photocopy or reproduce article content through the University of California Press’s Rights and Permissions website, http://www.ucpressjournals.com/
reprintInfo.asp. DOI: 10.1525/cond.2009.090062
Resumen. muchos estudios han demostrado los efectos ne-
gativos de los depredadores introducidos sobre las poblaciones
de especies de aves residentes en islas. Sin embargo, pocos han
cuantificado sus efectos sobre la supervivencia y el compor-
tamiento del uso del espacio de las especies migratorias. Usa-
mos radio-telemetría para investigar la supervivencia durante el
invierno y los patrones de comportamiento de descanso de Ca-
tharus bicknelli en dos sitios en la República Dominicana. La
PREDATION OF A WINTERINg mIgRATORy SONgbIRD by INTRODUCED RATS:
CAN NOCTURNAL ROOSTINg bEHAVIOR SERVE AS PREDATOR AVOIDANCE?
Abstract. many studies have demonstrated the deleterious
effects of introduced predators on resident populations of island
birds, but few have quantified their effect on the survival and
space-use behavior of migratory species. We used radio telem-
etry to investigate the winter survival and roosting patterns of
bicknell’s Thrush (Catharus bicknelli) at two sites in the Domin-
ican Republic. Depredation by introduced rats was the only cause
of mortality among 53 radio-tagged individuals monitored between
January and march over multiple years; five (9%) marked indi-
viduals were depredated. Predator trapping revealed the presence
of both the black rat (Rattus rattus) and Norway rat (R. norvegi-
cus) and that the density of rats was higher in broadleaf cloud
forest than in nearby pine forest. Some thrushes that used cloud
forest exclusively during the day roosted at night in adjacent pine
habitat. We suggest that introduced rats exert predation pressure
on wintering bicknell’s Thrush in the Dominican Republic and
that nocturnal arboreal rat predation could influence the thrush’s
space-use strategies.
5E-mail: jatownse@syr.edu
JaSon m. townSend1,5, ChriStopher C. rimmer2, JorGe BroCCa3, kent p. mCFarland4,
and andrea k. townSend4
1SUNY-College of Environmental Science and Forestry, Department of Environmental and Forest Biology, State University of New York,
1 Forestry Drive, Syracuse, NY 13210
2Vermont Center for Ecostudies, P. O. Box 420, Norwich, VT 05055
3Sociedad Ornitología de la Hispaniola, Parque Zoologico Nacional—ZOODOM, Avenida de la Vega Real, Arroyo Hondo,
Santo Domingo, Dominican Republic
4Fuller Evolutionary Biology Program, Cornell University Laboratory of Ornithology, Ithaca, NY 14850
Key words: Bicknell’s Thrush, Catharus bicknelli, Dominican
Republic, introduced predators, migratory songbirds, predation,
Rattus, roosting behavior.
manuscript received 6 April 2009; accepted 12 June 2009.
The Condor 111(3):565–569
 The Cooper Ornithological Society 2009
depredación por ratas introducidas fue la única causa de mortali-
dad en los 53 individuos marcados con transmisores que fueron
monitoreados entre enero y marzo por varios años; cinco (9%)
individuos marcados fueron depredados. Las capturas revelaron
la presencia de Rattus rattus y R. norvegicus y que la densidad de
ratas era más alta en los bosques siempre-verde de neblina que en
los bosques de coníferas cercanos. Algunas aves que utilizaron
los bosques de neblina sólo durante el día, descansaron durante la
noche en los hábitats de coníferas adyacentes. Sugerimos que las
ratas introducidas ejercen una presión de depredación sobre las
poblaciones invernantes de C. bicknelli en la República Domini-
cana y que la depredación nocturna por ratas arbóreas podría in-
fluenciar las estrategias de uso del espacio de esta especie.
Predator avoidance has been proposed as one of several fac-
tors that influence space-use strategies of migratory songbirds
on their nonbreeding grounds (Powell 1980). Relatively few
studies have investigated the effect of predators on wintering
songbirds explicitly, but at least two studies have supported the
idea that predation exerts a proximate influence on nonbreeding
birds’ space-use strategies (Winker et al. 1990, Cuadrado 1997).
Other factors influencing nonbreeding birds’ use of space include
the relative abundance and seasonal patterns of food resources
(Johnson and Sherry 2001), intraspecific dominance hierarchies
that result in the sexes segregating by habitat (marra et al. 1993),
and interspecific competition (greenberg 1986).
On Caribbean islands, where native predators are less com-
mon than on the mainland (Vitousek 1988), several studies of mi-
grants’ ecology have discussed the influence of avian predators on
their winter space use and roosting behavior (Latta and Faaborg
2001, brown and Sherry 2008, Smith et al. 2008). To our knowl-
edge, however, none have evaluated the potential effects of invasive
mammalian predators. In the Caribbean, such potential predators
include the feral cat (Felis catus), Indian mongoose (Herpestes
javanicus), Norway rat (Rattus norvegicus), and the highly arbo-
real black rat (R. rattus). Although the extent of predation pressure
on nonbreeding birds can be exceedingly difficult to determine
(Sherry and Holmes 1996), many studies of resident populations
of island birds have shown the extremely deleterious effects of in-
vasive mammalian predators (e.g., Atkinson 1985, Robinet et al.
Depredación durante la Invernada de Aves Canoras
migratorias por Ratas Introducidas:
¿El Comportamiento de Descanso Nocturno Sirve para Evitar
a los Depredadores?
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566 SHORT COmmUNICATIONS
1998, Nelson et al. 2002). Within the Caribbean, several studies
have found that introduced mammals depredate nests, fledglings,
and adults of both ground-dwelling and arboreal resident birds
(Seaman and Randall 1962, Arendt 2000, Townsend et al. 2008).
To date, however, there has been no documentation of predation by
introduced mammals on overwintering migratory species.
We used radio telemetry to monitor the daily movements,
survival, and nocturnal roosting behavior of wintering bick-
nell’s Thrushes (Catharus bicknelli) at two geographically and
ecologically distinct sites in the Dominican Republic. bicknell’s
Thrush is considered one of the nearctic–neotropical migrants of
highest conservation concern (Pashley et al. 2000, Wells 2007),
is classified as globally “vulnerable” by the International Union
for the Conservation of Nature (birdLife International 2000),
and is experiencing population declines in parts of its breeding
range (Campbell et al. 2008, Lambert et al. 2008). Additionally,
we used live mark–recapture and removal trapping of rats to col-
lect preliminary data on their relative population densities in dif-
ferent habitats used by bicknell’s Thrush at these two sites. Our
goals were to (1) quantify the extent and causes of mortality of
radio-tagged bicknell’s Thrushes, (2) quantify nocturnal densi-
ties of rats in different habitats used by bicknell’s Thrush, and (3)
compare patterns of nocturnal rat density with diurnal movement
and nocturnal roosting patterns of bicknell’s Thrush.
mETHODS
We conducted radio telemetry at two 10–15 ha sites in the Do-
minican Republic during four consecutive winters from 2005 to
2008. The two sites are located in the southwestern and north-
eastern corners of the country, approximately 130 km apart. The
southwestern site, Pueblo Viejo (hereafter “PUVI”), consists
of primary, undisturbed montane cloud forest, both pine forest
and hardwood forest, at 1600–1800 m elevation in the Sierra de
bahoruco (18° 12′ N, 71° 32′ W). The hardwood forest is char-
acterized by a dense understory of vine tangles from trees felled
in storms, complete canopy cover with trees reaching heights
between 15 and 20 m, and an abundance of lianas and epiphytes
(Veloz 2007). The pine forest is composed mainly of tall, straight
Hispaniolan pines (Pinus occidentalis) with a sparse understory
and few vines. At this site, the transition between pine and broad-
leaf forest is characterized by an abrupt ecotone of mixed forest
<25 m in width. The northeastern site, Loma La Canela (hereaf-
ter “LOCA”), is located in primary and secondary rainforest at
350–600 m elevation in the Cordillera Septentrional (19° 25′ N,
70° 8′ W). The site is characterized by a more open understory,
85–90% canopy cover with trees 15–25 m tall, and some of the
highest precipitation on the island (3000–4000 mm annually) as
a result of orographic effects and the prevailing northeast winds
(Sanchez and Hager 1997).
We captured bicknell’s Thrushes in 6- and 12-m 36-mm-
mesh mist nets, primarily by playback of the species’ vocaliza-
tions, and fitted all captured birds with 1.2-g radio transmitters
(model bD-2g, Holohil Systems, Ltd.), using the backpack-
harness method of Rappole and Tipton (1991). We checked all
birds for full leg and wing movement before release. We sexed
the birds by DNA extraction and amplification by the polymerase
chain reaction (griffiths et al. 1998). We aged the birds by the
shape of their rectrices (Collier and Wallace 1989). Using hom-
ing techniques (White and garrott 1990), we located radio-
tagged individuals with Wildlife materials TxR-1000 receivers
and 3-element hand-held yagi antennas. To determine homing
locations we approached the birds quietly and recorded points
on hand-held global Positioning System units (garmin gPSmap
76) with an average accuracy of ±8 m. We located the noctur-
nal roosting sites of a subset of birds. We tracked the birds over
a 4–6 week period between 1 January and 10 march each win-
ter. We used the Animal movement extension to ArcView 3.2 to
model core diurnal territories with fixed-kernel use distributions
(Hooge and Eichenlaub 2000).
We captured rats at PUVI during 2006 and at both PUVI
and LOCA during 2008 and 2009. At PUVI in 2006, we deployed
Sherman live traps in two sessions, one in January and one in
February. Traps were open for three consecutive nights during
each session. We placed 25 traps at 25-m intervals in broadleaf
cloud forest and adjacent pine forest. Traps were baited with a
mixture of oats and peanut butter placed both inside the trap and
immediately outside its entrance. Captured rats were ear-tagged
with a uniquely numbered aluminum band and released. In Feb-
ruary 2008 and February 2009, we deployed 25 snap traps in both
pine and broadleaf forest at PUVI and in mid-elevation rainfor-
est at LOCA. Traps were baited with oats and peanut butter, and
all dead rats were removed the following morning. In 2008 we
trapped at PUVI for two consecutive nights in one pine forest
and two cloud forest plots, in 2009 for three consecutive nights
in one pine forest and one cloud forest plot. At LOCA we trapped
for two consecutive nights in 2008 and four consecutive nights
in 2009.
STATISTICAL ANALySES
We determined trap success of both live trapping and snap trap-
ping, calculating it as the number of animals trapped per num-
ber of traps available, corrected for sprung traps (Engeman et
al. 2006). We examined proportion of captures as the response,
weighted by the total number of traps set, in a generalized linear
model (gLm) with habitat type (cloud forest vs. pine forest), trap
type (snap vs. live), and date as predictors, specifying binomial
errors and logit-link function in JmP v 7.0 (SAS Institute 2007).
Nonsignificant effects were removed from the final model. In a
separate gLm, we also compared trap success in rain forest at
LOCA vs. that in cloud forest at PUVI, again with proportion of
captures as the response and habitat as the predictor. There was
no evidence of overdispersion of data in either model. Parameter
estimates (β ± SE) are presented in logit scale. All other values
reported are means ± SD.
RESULTS
Of 53 bicknell’s Thrushes radio-tagged and monitored for at least
30 days during the winters of 2005–2008, we documented the
mortality of five (9.4%) (Table 1), all killed by introduced mam-
malian predators. All depredated individuals were male, and two
of the five were juveniles. All other radio-tagged birds survived
through the life of their transmitter’s battery or the termination of
our study. We recovered the carcasses and transmitters of all five
depredated thrushes. In each case, physical evidence strongly in-
dicated rat depredation. Four carcasses were found in shallow
underground tunnels with openings approximately 5 to 8 cm in
diameter from which we excavated their remains (sterna, tarsi,
wings, feathers, transmitters). These underground tunnels are
widespread at both sites and are known to provide underground
refuge for introduced rats (JmT pers. obs.). The fifth carcass was
found on the large horizontal limb of a broadleaf tree ~3 m above
ground, surrounded by fresh rat excrement.
In a single model with habitat and trap type as predictors,
trap success at PUVI was higher in broadleaf cloud forest than
in adjacent pine forest (β = 1.8 ± 0.4 cloud vs. pine; χ21 = 23.5,
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SHORT COmmUNICATIONS 567
birds visually, but on the basis of separation of the individuals’
transmitter signals these birds appeared to roost singly. During
the day, these individuals were never documented in pine forest
(>30 diurnal locations per individual). Overall, 68% of roost-
ing locations of border individuals were in pine forest (Table 2).
Some individuals roosted exclusively in pine, whereas others
roosted in pine as well as broadleaf (individual birds’ percent-
age of roosting sites in pines ranged from 31% to 100%; Table
2). These individuals traveled a mean distance of 62.1 m ± 38.6
to reach roosts in pines. When border individuals were not found
roosting in pine forest, they roosted either along the border be-
tween pine and broadleaf or within 12 m of their diurnal terri-
tories. Two of three individuals whose territories did not border
pine forest (“nonborder individuals”) and whose roosts we lo-
cated left their territories to roost in pine forest. One, a first-
winter male, traveled an average of 291.6 m to reach its roost in
pine forest, and the other, a first-winter female, roosted on 5 of
6 occasions in her core diurnal territory but spent one night in
pine forest 120.8 m from her core area. The third roosted in or
within 12 m of its diurnal territory. Searching pine forest for the
signals of other nonborder individuals produced no detections,
and, on the basis of weak signals in the direction of these areas,
we assumed that these birds roosted in or near their diurnal ter-
ritories. We did not, however, track these birds to exact loca-
tions because of difficulties of navigating dense cloud forest at
night. At LOCA, we found roost sites of three individuals on
one night each. All roosts were within these individuals’ diur-
nal territories. Nightly checking from a base camp in human-
disturbed habitat adjacent to the forest study plot did not yield
detections, only weak signals in the directions of diurnal terri-
tories within the forest.
DISCUSSION
To our knowledge, these data are the first to document depre-
dation of a migratory songbird on its nonbreeding grounds by
introduced predators. We confirmed depredation of bicknell’s
Thrushes in three of four winters of our study. In each incident,
the evidence strongly suggests rats as the primary predator, but
it is possible that the Indian mongoose was responsible for some
of the depredations. We believe this is unlikely, however, for sev-
eral reasons. First, the mongoose is rare in upper-elevation wet
habitats of the Caribbean such as our study sites (Horst et al.
2001), and we never captured any in our traps. Next, we rule out
the mongoose as the predator of the thrush found on a tree limb
~3 m above ground because in the Caribbean the mongoose is
not known to be arboreal at such heights (Nellis 1989) and be-
cause we found rat excrement around the carcass. Further, all
excavated tunnels from which we extracted the other four bick-
nell’s Thrush carcasses are known to be used by introduced rats
and had narrow openings (~5 to 8 cm in diameter), unlikely to be
large enough for the Indian mongoose, which makes wider bur-
rows in loose soil. Finally, the remains recovered in these holes
included sterna and tarsi with some flesh remaining, unlike the
wings- and feathers-only remains of a songbird depredated by
a mongoose and left in open habitat (Townsend 2006). We be-
lieve this evidence indicates rats as the responsible predators in
each case. Furthermore, we suggest that on Hispaniola rat popu-
lations, which have expanded since introduction ~500 years ago,
could reduce winter survival of migrant songbirds such as bick-
nell’s Thrush and influence their space-use strategies. Little is
known about current trends in rat populations on Hispaniola, and
our study is the first to monitor Hispaniolan rat populations in
cloud forest and rainforest.
P < 0.001) and was higher for snap traps than for live traps (β = 1.1 ±
0.3 snap vs. live; χ21 = 13.8, P < 0.001). In a separate model com-
paring cloud forest at PUVI and rain forest at LOCA, trap success
at these ecologically and geographically distinct sites did not dif-
fer (gLm, β = −0.3 ± 0.3 cloud vs. rain; χ21 = 1.0, P = 0.32). mean
trap success in cloud forest by live trapping was 12.0% ± 2.9%,
whereas in pine forest it was 3.5% ± 2.9% (Fig. 1). mean trap suc-
cess in cloud forest by snap trapping was 34.0% ± 7.9%, whereas
in pine forest it was 4.6% ± 0.6% (Fig. 1). At LOCA, where we used
only snap traps, mean trap success was 39.1% ± 2.8% (Fig. 1.) At
all sites and by both trapping methods, we captured both black and
Norway rats, but not all workers could distinguish the two species.
We cannot, therefore, summarize their relative numbers and do not
know which species was responsible for depredating thrushes.
At PUVI during winter 2005, we located nocturnal roosts
(n = 75) of 12 individual bicknell’s Thrushes with discrete,
minimally overlapping diurnal territories in cloud forest. Of
these, nine maintained cloud-forest territories immediately ad-
jacent to the transition to pine forest, so we refer to them as
“border individuals.” All nine border individuals roosted at
least occasionally in the canopies of pines, at estimated heights
of 10–15 m. We were unable to pinpoint the location of roosting
FIgURE 1. Rat-trap success per 100 corrected trap-nights for snap
trapping (n) conducted in multiple habitats in the Dominican Repub-
lic, 2008–2009. Cloud = cloud forest (n = 3), rain = rainforest (n = 2),
and pine = pine forest (n = 2). The middle horizontal bar represents
mean trap success; the upper and lower bars represent the maximum
and minimum values.
TAbLE 1. Number of bicknell’s Thrushes depredated by intro-
duced rats at a high-elevation cloud-forest site (Pueblo Viejo) and
a mid-elevation rainforest site (Loma La Canela) in the Dominican
Republic.
Site and year
birds
tracked (n)
birds
depredated (n)
Depredation
of tracked
birds (%)
Pueblo Viejo
2005 20 1 5.0
2006 16 3 18.8
Loma La Canela
2007 4 0 0.0
2008 13 1 7.7
Total
2005–2008 53 5 9.4
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568 SHORT COmmUNICATIONS
Our success rate with snap traps in both cloud forest and
rainforest (25–41%) is higher than that of studies of introduced
rats on islands in the Pacific, which generally report <10% trap
success (Tamarin and malecha 1972, Robinet et al. 1998). Our
success rate is comparable, however, to that reported from mid-
elevation wet forest in Puerto Rico’s Sierra de Luquillo (33–44%),
a rate among the highest known (Engeman et al. 2006). This
Puerto Rican forest is used by a wide range of migrants, includ-
ing bicknell’s Thrush (Rimmer et al. 2001).
Although during daylight hours rats may pose little threat
to highly mobile and alert songbirds, at night they may be more
successful in encountering immobile, roosting individuals. At
dusk we frequently observed rats climbing at all vertical levels
of the cloud forest and rainforest, from ground level to >15 m.
At PUVI the use of nocturnal roosts in pine forest might be a
space-use strategy to avoid nocturnally active and highly arbo-
real rats. At PUVI our rat-population indices show that numbers
of rats in broadleaf forest exceed those in pine forest by several
orders of magnitude. Pine forest is characterized by tall, straight
trees with few lower branches and an open understory, which to-
gether may impede arboreal movements of rats. broadleaf cloud
forest, in contrast, is characterized by extensive horizontal and
oblique branches and a dense understory of shrubs and vine tan-
gles, features that may facilitate the movement of rats through
the canopy. We suggest that birds that maintain diurnal terri-
tories in broadleaf forest but roost in adjacent pines may avoid
predators.
An alternative to the predator-avoidance explanation is
that birds roost nocturnally in a habitat better for thermoregu-
lation (Walberg and King 1980, merola-Zwartjes 1989), as has
been suggested for several species of songbirds wintering in the
Caribbean (e.g., Latta and Faaborg 2001, Smith et al. 2008). This
possibility is unlikely at our sites, however, where minimum am-
bient nighttime temperatures in pine forest averaged 1° C lower
than in cloud forest, significantly colder (Student’s t34 = 2.3, P =
0.02, JmT, unpubl. data). bicknell’s Thrushes that roost in pine
forest are therefore moving from the warmer and likely more
protected microclimate of dense cloud forest to a colder, more
exposed microclimate.
In contrast to the patterns observed at PUVI, at LOCA we
did not detect nocturnal roosting by bicknell’s Thrushes out-
side of diurnal territories, although our data for LOCA are not
extensive. There is no pine forest at or near this mid-elevation
rainforest site. Neighboring habitats include pasture, shifting
agriculture, agroforestry, and regenerating forest in various
stages of succession; none of these appeared to provide mi-
crohabitats suitable for roosting. It might be that LOCA does
not provide an alternative night refuge outside of diurnal ter-
ritories, with negative consequences for bicknell’s Thrush sur-
vival, given that here we trapped rats at a rate as high as in cloud
forest at PUVI.
Few other studies have investigated the benefits of habitat-
specific roosting behavior on birds’ nonbreeding grounds. Al-
though at least four species of migrants wintering in Puerto Rico
move from black mangrove (Avicennia germinans), white man-
grove (Languncularia racemosa), and dry forest to night roosts in
red mangrove (Rhizophora mangle), the proximate causes of this
specialization in roosting habitat have yet to be identified (Re-
itsma et al. 2002, burson et al. 2005, Smith et al. 2008). brown
and Sherry (2008) found that most Ovenbirds (Seiurus aurocap-
illa) wintering in dry forest in Jamaica roosted within their core
diurnal territories and suggested that this even spacing across the
landscape could confer an advantage in avoiding predators.
Although many factors influence the population dynam-
ics and space-use strategies of migratory birds on their winter
grounds, risk of mortality from introduced and naturally occur-
ring predators should be considered an important potential selec-
tion pressure. High levels of predation in certain winter habitats
could act in concert with other determinants of habitat quality
(e.g., food resources, precipitation, intraspecific aggression, par-
asite prevalence) to affect winter survival negatively, thereby in-
fluencing overall demographics and population viability. Further
study of introduced rat populations in multiple habitats on His-
paniola is needed to determine the extent and effects of predation
on both migratory and resident birds and the potential for correc-
tive management.
We gratefully acknowledge funding support from the Wilson Orni-
thological Society, Association of Field Ornithologists, Eastern bird
banding Association, macArthur Foundation, the Stewart Founda-
tion, the Thomas marshall Foundation, the U.S. Fish and Wildlife
Service, and the U.S. Forest Service International Program. Permis-
sion to band and place transmitters on birds was provided by the U.S.
geological Survey. Permission to conduct research in the Domini-
TAbLE 2. Number of nocturnal roost fixes, percentage of roosts in pine forest,
and distance from diurnal core use areas to roost sites for nine bicknell’s Thrushes
with diurnal territories in cloud forest bordering pine forest at Pueblo Viejo,
Dominican Republic, 2005–2006. m = male, F = female, Hy = hatching year, AHy =
after hatching year.
Transmitter Sex Age
Roost
fixes
(n)
Fixes in
pine (%)
mean distance
from roost to core
area ± SD (m)
133 m Hy 12 83.3 46.3 ± 34.5
170 m Hy 6 100 66.6 ± 40.9
106 m AHy 13 30.8 34.0 ± 3.3
267 m AHy 10 70 73.7 ± 55.1
437 m AHy 3 100 98.7 ± 27.3
734 m AHy 3 66.7 82.8 ± 43.5
938 m AHy 6 100 60.0 ± 20.4
899 F Hy 2 50 12.0
981 F AHy 11 36.4 83.3 ± 19.3
All — — 66 68.2 62.1 ± 38.6
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SHORT COmmUNICATIONS 569
can Republic was provided by the Subsecretaria de Áreas Protegidas
y biodiversidad. We are especially thankful to J. Klavins, S. Frey, E.
garrido, and V. mejia for their outstanding field work under difficult
conditions. The manuscript was greatly improved by the comments
of Robert Curry and Steven Latta.
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