ORNITOLOGIA NEOTROPICAL 18: 543–552, 2007
© The Neotropical Ornithological Society

ANTI-PREDATOR RESPONSES OF NEOTROPICAL RESIDENT
AND MIGRANT BIRDS TO FAMILIAR AND UNFAMILIAR OWL
VOCALIZATIONS ON THE YUCATAN PENINSULA
Matthew W. Reudink1,2,3, Joseph J. Nocera2, & Robert L. Curry1
1

Department of Biology, Villanova University, Villanova, PA, 19002, USA.
E-mail: reudinkm@biology.queensu.ca

2

Department of Biology, Queen’s University, Kingston, Ontario, K7L 3N6, Canada.

Resumen. – Comportamiento de evasión de depredadores de aves Neotropicales residentes y
migratorias a vocalizaciones de lechuzas familiares y no familiares en la Península de Yucatán. –
Para reducir el riesgo de depredación, los animales necesitan utilizar estrategias de detección de depredadores efectivas. Muchas especies dependen de su experiencia previa, o de aprendizaje, para detectar el
riesgo de depredación, pero esta estrategia es inefectiva cuando estas especies son expuestas a un depredador potencial que no les es familiar. Sin embargo, varias especies exhiben un comportamiento innato a
depredadores potenciales pero, hasta donde sabemos, ningún estudio ha examinado el comportamiento de
aves hacia especies alopátricas de depredadores que no son familiares. En el presente estudio expusimos
aves tropicales migratorias y residentes en la Península de Yucatán, México, a vocalizaciones de dos especies de lechuza: el residente Tecolote Bajeño (Glaucidium brasilianum), el cual se asume que es familiar a
ambos grupos migratorios y residentes, y un autillo alopátrico norteamericano Tecolote Chillón (Megascops
asio), el cual probablemente solo es familiar al grupo migratorio que se reproduce en zonas templadas del
este de Norteamérica. La mayoría de aves únicamente respondieron a las vocalizaciones de depredador que
se esperaba fuera familiar: ambos grupos migrantes y residentes respondieron vigorosamente al tecolote
bajeño; el autillo solo elicitó comportamiento de las aves migrantes. Estos resultados indican que las aves
residentes no tienen la habilidad de reconocer de forma innata, o de deducir peligro, de las vocalizaciones
de depredadores nuevos. Así mismo que se expanden las distribuciones de especies, y depredadores exóticos se introducen en áreas nuevas, aumentará en importancia el entendimiento de capacidad y estrategias
de reconocimiento de depredadores en especies de presa.
Abstract. – To reduce predation risk, animals must employ effective predator-detection strategies. Many
species rely on prior experience, or learning, to detect predation risk, but this strategy is ineffective when
exposed to an unfamiliar potential predator. Many species, however, exhibit an innate response to potential
predator species but to our knowledge, no studies have examined the response of birds to unfamiliar, allopatric predator species. In this study, we exposed migrant and resident tropical birds on the Yucatan Peninsula, Mexico, to vocalizations of two owl species: the resident Ferruginous Pygmy-owl (Glaucidium
brasilianum), assumed to be familiar to both migrants and residents, and the allopatric Eastern Screech-owl
(Megascops asio), likely to be familiar only to migratory birds that breed in eastern temperate North America.
Most birds responded only to predator vocalizations with which they were expected to be familiar: both
migrants and residents responded strongly to Ferruginous Pygmy-owl vocalizations; the Eastern Screechowl vocalization elicited a response only from migrants. These results suggest that tropical residents are
______________
3

Current address: Department of Biology, Queen’s University, Kingston, Ontario, K7L 3N6, Canada.

543

REUDINK ET AL.

unable to recognize innately, or to infer danger from, vocalizations of novel predators. As species ranges
expand and exotic predators are introduced to new areas, it will be important to better understand the
predator recognition response of potential prey species. Accepted 31 July 2007.
Key words: Anti-predator, migrant, mobbing, playback, Ferruginous Pygmy-owl, Glaucidium brasilianum.

INTRODUCTION
Predator detection is crucial for reducing an
animal’s predation risk (Lima & Dill 1990,
Caro 2005). Many species rely on experience
with a predator to facilitate recognition
(Mirza et al. 2006). However, innate recognition of predators has been demonstrated
repeatedly in taxa as diverse as desert pocket
mice (Chaetodipus pencillatus, Punzo 2005),
Atlantic salmon (Salmo salar, Hawkins et al.
2004), American toads (Bufo americanus, Gallie
et al. 2001) and many species of birds (e.g.,
Veen et al. 2000, Goth 2001, Wiebe 2004).
Notably, these and other similar studies have
examined responses by prey to only sympatric
species. Very few studies (e.g., Blumstein et al.
2000, Veen et al. 2000) have examined the
response to an allopatric, unfamiliar predator
species.
Some prey species respond to visual stimuli of a foreign predator (Blumstein et al.
2000, Veen et al. 2000). However, predators
are not always visible; sometimes the only evidence of their presence may be auditory.
Some species recognize and respond to audible cues produced by predators (e.g., birds:
Miller 1952, Hauser & Caffrey 1994,
Hakkarainen et al. 2002, Rainey et al. 2004, and
mammals: Swaisgood et al. 1999, Gil-da-Costa
et al. 2003). To our knowledge, only the study
by Blumstein et al. (2000) has examined the
response to auditory cues of allopatric, unfamiliar predators: in that study, tammar wallabies (Macropus eugenii) responded to visual, but
not auditory, stimuli of both novel and familiar predatory species. The generality of this
result to other animal groups remains to be
tested. This is essential to our understanding
544

of the mechanisms of predator recognition:
we need to know which cues are pertinent to
certain animal groups and the circumstances
under which they are important.
When encountering potential predators,
individuals can restrict their response to only
those species with which they are familiar.
Conversely, they may use internal cues (autonomous neuropsychological processes) to presume an unfamiliar predator species is
dangerous. Indeed, such internal cues have
helped some predator-naïve populations of
red-necked pademelons (Thylogale thetis) to
survive introductions of exotic predators
(Blumstein et al. 2002). We predict that if individuals need to maximize awareness of predatory hazards, most should use internal cues to
respond to threat signals regardless of previous experience, and few will restrict their
responses to only those signals with which
they are familiar. To test this prediction, we
deployed songs of a familiar and an unfamiliar
predatory owl species and monitored the
response of over-wintering migrant and resident Neotropical birds at a study site in the
Yucatan Peninsula of Mexico. We chose to
use auditory cues of owls because they
generally vocalize en route to their hunting
perches, providing a reliable indicator of local
owl activity (Hendrie et al. 1998). These auditory cues are also appropriate because small
owls tend to produce high-pitched staccato
vocalizations (Miller 1934, 1947), which
makes members of this group generally easy
to recognize by sound and creates appropriate
circumstances to detect responses from internal cues. This design avoids potential biases
created by using entirely out-of-context
sounds.

ANTIPREDATOR RESPONSES OF RESIDENT AND MIGRANT BIRDS

We used the Eastern Screech-owl (Megascops asio) as our allopatric exemplar. It is a
small owl that commonly depredates passerines (Gehlbach & Leverett 1995). It is widespread east of the Rocky Mountains from
Canada to northern Mexico (Gehlbach 1995).
The Eastern Screech-owl’s range does not
extend south into the Yucatan Peninsula and
its song would therefore be a novel vocalization to the resident species of the Yucatan.
Eastern Screech-owls have two distinct songs,
a monotonic trill and a descending trill (Gehlbach 1995). Passerines that breed in eastern
North America (including the majority of
migratory birds at our study site) should be
familiar with the vocalization and respond to
the screech-owl’s monotonic trill (Gehlbach &
Leverett 1995).
The Ferruginous Pygmy-owl (Glaucidium
brasilianum) is a small owl that is also an
important predator of passerines (Proudfoot
1997); it is abundant and distributed widely
across the tropical lowlands of Mexico
(Proudfoot & Johnson 2000). Ferruginous
Pygmy-owls are numerous and conspicuous at
our study site, so we are confident most birds
in the area had been exposed to their song.
The Ferruginous Pygmy-owl’s song (also
described as a call, e.g., Proudfoot & Johnson
2000) is a series of hollow whistles, sometimes
ending with high, yelping twitters (Howell &
Webb 1995); it is imitated commonly by birders both to attract pygmy-owls and elicit a
response from songbirds and has been used
to assist point-counts in the Yucatan (Lynch
1989).
If resident birds of the Yucatan can recognize vocalizations innately, then there should
be no difference in their response to the unfamiliar screech-owl and the familiar pygmyowl. This is the same pattern we would expect
from Neotropical migrants overwintering in
the area, who should be familiar with both.
Conversely, if resident birds of the Yucatan do
not innately recognize predator vocalizations,

then there should be a biased response toward
the familiar pygmy-owl.

METHODS
To test our predictions, observations and
playbacks were conducted from 3–7 March
2004 in deciduous dry forest adjacent to the
Centro Ecologico Akumal in Akumal,
Yucatan, Mexico. Four study sites, roughly
200 m apart, were chosen for this experiment
based on vegetation composition and accessibility. This distance is adequate to provide us
with independent samples; several studies
(e.g., Sliwa & Sherry 1992, Confer & Holmes
1995) have shown that sites separated by
>100 m effectively eliminated double-counting in vegetated Neotropical habitats, and in
such habitats, sound from playbacks (~75
Db) should quickly attenuate to background
noise by 100 m. Two sites were within forest,
while the other two were in a 15 m-wide telephone line right-of-way dominated by scrub
and edge vegetation.
Experiments consisted of providing owl
song playback for 150 s followed by 30 s of
silence. This pattern was repeated three additional times for each trial. Each playback was
preceded with 2 min of silence and followed
with 7 min of silence. Total playback trial
duration was 21 min. The Eastern Screechowl playback was of a 1 min recording of their
monotonic trill (cycled continuously). The
Ferruginous Pygmy-owl playback was of a 20
s recording of their repeated song (cycled
continuously). Because our study design only
incorporated one repeated song for each playback, we used linear mixed-effects models
(see below) to reduce the effect of potential
pseudo-replication by accounting for random
error associated with sampling design. We
played songs at a constant volume using an
iPod mini (Apple Computer, Inc.) attached to
a speaker (Saul Mineroff Electronics, Inc.) on
the ground in the site center. Because of a lack
545

REUDINK ET AL.

of control, we tested only the directionality of
response, rather than the strength of the
response.
Playbacks were alternated between sites,
and were restricted to 06:00–12:00 and 14:00–
17:00 h, to capture the peak activity period
(Poulsen 1996). Each day, the order of
sites visited and songs played varied from
the previous day so that all sites were visited
in a different order and time of day. Overall,
15 playback trials were conducted using Eastern Screech-owl song and 15 using Ferruginous Pygmy-owl song (edge site 1: 4 screechowl, 3 pygmy-owl; edge site 2: 4 screechowl, 4 pygmy-owl; forest site 1: 4 screech-owl,
4 pygmy-owl; forest site 2: 3 screech-owl, 4
pygmy-owl).
All individuals responding to playbacks
were recorded throughout the trial, including
the pre- and post-treatment silence periods.
We considered birds to be responding if they
exhibited the following behaviors: agitation,
chipping, calling, flying in to the speaker, or
attacking a Ferruginous Pygmy-owl that had
responded to the playback (which occurred in
73% of the trials in which a Ferruginous
Pygmy-owl song was played). Those birds
passing through the site without responding
or showing interest in the speaker were
deemed “passers-by” and not used in the subsequent analyses.
Because most observed birds were
actively responding to the playback, we could
not use a binary response variable (responding, not responding) to control for species
abundance. Instead, we built linear mixedeffects models (LME), using R v. 2.1.1 (R
Development Core Team 2005) to assess
whether the absolute number of respondent
migrants and residents differed between owlsong treatments (α = 0.05), and the direction
of those relationships. The psuedoreplicated
structure of our experiments yields a small
effective sample size. To contend with this,
we chose LME for our analyses because it
546

accounts for the non-independence of errors
created by spatial and temporal pseudoreplication. LME simultaneously estimates how
variables influence the mean (fixed effects)
and predicts how within-group correlation
(random effects related to sampling design,
such as the use of playback song from a single
individual) influences the variance (Crawley
2005). As predictive variables, we used a
binary treatment classification (Eastern
Screech-owl vs Ferruginous Pygmy-owl) as a
fixed effect, and included time-of-day, nested
within site (to weight for within-group
errors), as a random effect. We compared
variance components to assess model fit.
We differentiated between migrant and
resident species with the classifications used
by Lynch (1989). We then tested for detectable differences in the number of individual
migrants (“all-migrant” model) vs residents
(“all-resident” model). To examine patterns
more closely, we then built species-specific
models for the most commonly detected species: Yucatan Jay (Cyanocorax yucatanicus; resident), Yucatan Vireo (Vireo magister; resident),
Mangrove Vireo (V. pallens; resident), Whiteeyed Vireo (V. griseus; migrant), and Melodious Blackbird (Dives dives; resident).
We expected that if a playback type was
associated with a greater number of individuals in the all-migrant and all-resident models,
it should also be associated with a greater
number of species. In other words, within
each migratory group (i.e., migrants or residents), we sought to determine which song
elicits a response from the most species.
Because we lacked the appropriate degrees of
freedom to construct LME models (migrant
species abundance per trial ranged from 0–3),
we instead used Wilcoxon non-parametric
tests to examine this; if our expectation were
not supported (indicating the medians were
drawn from the same distribution), it would
reveal important species-specific bias in our
results.

ANTIPREDATOR RESPONSES OF RESIDENT AND MIGRANT BIRDS

TABLE 1. Number of individuals of each species responding to playbacks from Eastern Screech-owls
(EASO) and Ferruginous Pygmy-owls (FEPO). Migratory status of each species is denoted as M (Migrant)
or R (Resident), as classified by Lynch (1989).
Common names
Black Catbird
Black-cowled Oriole
Blue Bunting
Buff-bellied Hummingbird
Canivet's Emerald
Caribbean Elaenia
Cinnamon Hummingbird
Common Yellowthroat
Golden-fronted Woodpecker
Gray Catbird
Great-tailed Grackle
Hooded Oriole
Great Kiskadee
Magnolia Warbler
Mangrove Swallow
Mangrove Vireo
Melodious Blackbird
Orange Oriole
Ovenbird
Rose-throated Becard
Rufous-browed Peppershrike
Social Flycatcher
Spot-breasted Wren
Tropical Kingbird
Tropical Mockingbird
White-eyed Vireo
White-winged Dove
Yellow-throated Warbler
Yucatan Jay
Yellow-lored Parrot
Yucatan Vireo

Scientific names
Melanoptila glabrirostris
Icterus prosthemelas
Cyanocompsa parellina
Amazilia yucatanensis
Chlorostilbon canivetii
Elaenia martinica
Amazilia rutila
Geothlypis trichas
Melanerpes aurifrons
Dumetella carolinensis
Quiscalus mexicanus
Icterus cucullatus
Pitangus lictor
Dendroica magnolia
Tachycineta albilinea
Vireo pallens
Dives dives
Icterus auratus
Seiurus aurocapilla
Pachyramphus aglaiae
Cyclarhis gujanensis
Myiozetetes similis
Thryothorus maculipectus
Tyrannus melancholicus
Mimus gilvus
Vireo griseus
Zenaida asiatica
Dendroica dominica
Cyanocorax yucatanicus
Amazona xantholora
Vireo magister

RESULTS
Few individuals were detected in the preplayback period (18 total) and no individuals were behaving in such a way as to
be considered “respondents” prior to playbacks. Individuals present in the pre-playback period were therefore only included
in subsequent analyses if they flew in toward
the speaker and a mobbing response was

EASO
0
0
3
0
0
0
2
0
0
1
0
1
5
4
0
1
2
0
1
0
0
0
1
0
0
5
0
0
2
0
0

FEPO
1
1
1
2
1
1
8
1
2
1
2
3
1
5
1
12
12
1
0
2
1
11
3
4
1
10
2
2
46
2
31

Status
R
R
R
R
R
R
R
M
R
M
R
R
R
M
R
R
R
R
M
R
R
R
R
R
R
M
R
M
R
R
R

elicited. Of the 170 resident birds that
responded to playback, 152 birds (representing 27 species) responded to Ferruginous
Pygmy-owl song and 18 birds (9 species)
responded to Eastern Screech-owl song. Of
the 31 migratory birds that responded to
playback, 11 (5 species) responded to
Eastern Screech-owl song, while 20 (6
species) responded to Ferruginous Pygmyowl song. All species responding to play547

REUDINK ET AL.

TABLE 2. Linear mixed-effects models describing abundance of migrants, residents, and the five species
most commonly detected at playbacks of Eastern Screech-owl and Ferruginous Pygmy-owl songs in Akumal, Mexico. As predictive variables, a binary treatment classification (screech-owl vs pygmy-owl) was used
as a fixed effect, and time-of-day (nested within site) was used as a random effect. Comparison of SD of
the random-effect and intercept-only models estimates the time-of-day influence. T-value indicates
strength of relationship between abundance and treatment type; except for White-eyed Vireo, abundance
in all models was positively related to playbacks of Ferruginous Pygmy-owl.
Species/group
All-migrants
All-residents
Mangrove Vireo
Melodious Blackbird
White-eyed Vireo
Yucatan Jay
Yucatan Vireo

SD intercept-only
1.29
2.16
0.37
0.94
0.01
0.99
1.06

backs and their migratory status are listed in
Table 1.
The overall number of residents was
strongly related to playback type; however,
response by migrants was less marked (Table
2). All residents were positively associated
with playbacks of Ferruginous Pygmy-owls.
This relationship is especially apparent in the
resident species-specific models for Mangrove Vireo, Melodious Blackbird, Yucatan
Jay, and Yucatan Vireo (Table 2). Migrant
response did not differ between playback
types (Figure 1); however the White-eyed
Vireo was positively associated with Eastern
Screech-owl playback in the species-specific
model (Table 2). The random effects of timeof-day were negligible across sites for both
the all-resident and all-migrant models compared to the intercept-only model (Table 2),
therefore there is no important effect of
within-group (time-of-day | site) error from
temporal and spatial pseudoreplication on the
covariance structure of the data (i.e., errors
are independent).
There was no significant difference in the
number of migrant species responding to Ferruginous Pygmy-owl playbacks as compared
to Eastern Screech-owl playbacks (Wilcoxon
548

SD random-effect model
0.001
0.0003
0.0005
0.001
0.0002
0.001
0.001

t-value
1.94
9.50
4.51
2.05
1.91
2.45
9.19

P
0.06
< 0.0001
0.0001
0.05
0.07
0.02
< 0.0001

test: Z = -1.348, P = 0.18). The number of
resident species responding to Ferruginous
Pygmy-owl playbacks was significantly higher
than the number of resident species responding to Eastern Screech-owl playbacks (Wilcoxon test: Z = -4.278, P < 0.001; Fig. 1).

DISCUSSION
Our results reveal an important component
of the predator recognition process: birds
responded to vocalizations of predator species that they recognized. Migrant and resident tropical birds responded to familiar
songs of the Ferruginous Pygmy-owl. However, migrants responded more strongly to
the vocalizations of the Eastern Screech-owl
than did tropical resident birds.
Because they produce distinctive vocalizations (Miller 1934, 1947), most small
owls can be recognized audibly. Accordingly,
it could be expected that Neotropical residents would respond equally to the familiar pygmy-owl and the novel screechowl songs. However, our results do not
support this expectation. Instead our
results suggest that birds tend to respond
to threat cues with which they are experi-

ANTIPREDATOR RESPONSES OF RESIDENT AND MIGRANT BIRDS

FIG 1. Box plots of the number of individuals (A)
and species (B) responding to Eastern Screech-owl
(n = 15) and Ferruginous Pygmy-owl (n = 15)
playbacks. Gray boxes represent Neotropical
migrants, open boxes represent species resident to
the Yucatan Peninsula. Box plots show the
median, 10th, 25th, 75th, and 90th percentiles with
horizontal lines. Data points outside this range are
represented by open circles.

enced, and do not infer danger from unfamiliar sounds.
An alternate explanation for our results,
which our data cannot refute, is that the Neotropical residents did not respond to the
screech-owl playback due to ‘neophobia’ (e.g.,
fear of a new sound in the area). This explanation is plausible because migrants tend to
show greater exploratory behavior and less
neophobia than non-migrants (Mettke-Hofmann & Gwinner 2004). However, this is a
less parsimonious explanation than our argu-

ments regarding recognition; susceptibility to
neophobia differs between species even
within closely related taxonomic groups
(Mettke-Hofmann et al. 2002, Reader 2003)
and it therefore is unlikely to simultaneously
afflict such a large group of species as resident
birds of the Yucatan.
Resident bird species have presumably
never had contact with an Eastern Screechowl and therefore have not learned the level
of threat it poses. The finding that some
resident species did respond to the Eastern
Screech-owl playback (Table 1), however,
suggests that some species may have an innate
response to owl vocalizations, or aspects of
the vocalizations shared with familiar owl
species, and warrants further investigation.
One explanation that we cannot rule out,
and would be clarified by additional playback
studies, is that the Eastern Screech-owl
song may be similar enough to that of a
resident congeneric species, the Vermiculated Screech-owl (M. guatemalae), to elicit
an anti-predator response. Vermiculated
Screech-owls, whose songs also constitute
monotonic trills, are widespread in the
Yucatan (Howell & Webb 1995) and we
observed several within 5 km of our study
site, although we did not detect any at the site
itself.
Although we provide evidence that birds
in our study only responded to predator
vocalizations with which they were familiar,
the generality of this finding is unclear.
Mixed-species flocks in tropical forests often
vary in size, composition, and dynamics
depending on latitude and habitat type
(Poulsen 1996). Undoubtedly, other communities may show different response patterns
because they face predation threats that exert
different types of selective influence. For
instance, many tropical environments have
pronounced dry and wet seasons causing several species to move widely between seasonal
resource patches (Fogden 1972, Thiollay
549

REUDINK ET AL.

2002), which creates a constantly changing
predation landscape. Predator recognition in
these systems may require a different process
than we observed in the Yucatan.
Another possible explanation is that the
initiation of a mobbing response by migrant
species may trigger a mobbing response by
the residents, as observed among Blackcapped Chickadees (Poecile atricapillus, Hurd
1996, Turcotte & Desrochers 2002) and other
species (Forsman & Mönkkönen 2001). Our
data do not allow investigation of this hypothesis or to estimate the role and direction of
information transfer between heterospecifics; one group of species may respond more
or less eagerly when parasitizing the information (e.g., mobbing behavior indicating a
predator’s location) provided by heterospecifics in another group. However, playbacks can
be a valuable tool for elucidating individual
species’ roles in mixed-species flocks
(Goodale & Kotagama 2005). The migratory
White-eyed Vireo was the most common
respondent to Eastern Screech-owl playbacks,
responding in 33% of trials. Although a common respondent, we found no evidence from
the literature that White-eyed Vireos (or any
species for which we constructed separate
models) act as sentinel species, though we
suggest its potential role in mobbing response
warrants further investigation.
We have shown that many bird species in
the Yucatan failed to recognize the sounds of
an exotic predator. However, other studies
have demonstrated that some animals can
respond appropriately to exotic predatory
threat (Blumstein et al. 2002). This highlights
the importance of identifying the species
most vulnerable to invasive or introduced
predators, a particularly common phenomenon in the tropics. There is ample opportunity to study this, even outside dynamic
tropical communities. For instance, the
Barred Owl (Strix varia) is rapidly expanding
its range westward in North America (Dark et
550

al. 1998), creating not only competition for
the congeneric Spotted Owl (S. occidentalis),
but also constituting a novel predatory threat
to small bird and mammal species in areas it
invades. We predict that some species will
effectively discriminate between, and respond
to, such sympatric and evolutionarily allopatric predators.

ACKNOWLEDGMENTS
We acknowledge G. A. Proudfoot for insightful comments on an earlier version of this
manuscript. Reviews and suggestions from R.
McNeil, M. Mönkönnen, and three anonymous referees improved the final version. M.
Dappen, T. Fitzgerald, K. Mathot, and G.
Riecau also provided valuable discussion. We
also thank D. Mennill for the use of his
recordings of Ferruginous Pygmy-owls and
the Centro Ecologico Akumal for permission
to work on their land.

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