Behavioral Ecology
doi:10.1093/beheco/ari012
Advance Access publication 24 November 2004
Birds associate species-specific acoustic and
visual cues: recognition of heterospecific rivals
by male blackcaps
Piotr Matyjasiak
Centre for Ecological Research PAS, PL-05-092 Lomianki, Poland, and Laboratoire de Parasitologie
Évolutive, CNRS-UMR 7103, Université Pierre et Marie Curie, 7 quai St. Bernard, Bâtiment A 7ème
étage, Case 237, F-75252 Paris Cedex 05, France
Animals need to be able to identify other species, which is crucial in competition for ecological resources, for using other species
as a cue in habitat selection, and for the establishment and maintenance of parasite-host species associations in brood-parasitic
species. The ability to discriminate between conspecifics and heterospecifics may be crucially important also in refinement of
mating preferences during speciation and for premating isolation of sympatric species. It has long been hypothesized that species
identification ability is based on learned associations between multiple features that distinguish species. Here I test this
hypothesis using dual-choice song playback experiments with interspecifically territorial male blackcaps (Sylvia atricapilla) that
defend territories against male garden warblers (Sylvia borin). I show that male blackcaps can associate species-specific songs with
species-specific plumage and also that they retain the memory of this association for an 8-month period without contact with
heterospecific rivals. Apparently, yearling male blackcaps achieve this ability several months prior to their first breeding attempt.
This is the first time a long-term memory of associations between species-specific signals from two different sensory modalities
(visual and auditory) has been shown to be important for distinguishing conspecifics from heterospecifics. Key words: birds, longterm memory, playback experiments, song, species recognition, territorial behavior. [Behav Ecol 16:467–471 (2005)]
nimals living in complex communities need to be able to
identify other species and retain memories of species
identities for long periods of time to make optimal choices. In
particular, correct species recognition is important in competition for ecological resources (Martin PR and Martin TE,
2001a,b; Reed, 1982), for the use of resident species as a cue
in habitat selection by migratory species (Mönkkönen et al.,
1996), and for the establishment and maintenance of parasitehost species associations in brood-parasitic birds (Payne et al.,
2000; Slagsvold and Hansen, 2001). In the case of interference competition, this ability allows a potential threat posed
by individuals of other species to be more easily recognized
(Martin et al., 1996; Temeles, 1994) and the resource to be
more effectively defended (Ewald and Bransfield, 1987; Loyn
et al., 1983). The ability to discriminate between conspecifics
and heterospecifics may also crucially influence mate choice
decisions or sexual rival recognition (Hansen and Slagsvold,
2003; Ratcliffe and Grant, 1983, 1985; Searcy and Brenowitz,
1988; Slagsvold et al., 2002) and may be important in
refinement of mating preferences during speciation and for
premating reproductive isolation of sympatric species (Baker
MC and Baker AEM, 1990; Grant BR and Grant PR, 1996,
1998; Irwin et al., 2001; Price, 1998). Species identification
coupled with other categories of recognition has important
implications for the evolution of more sophisticated forms of
cognition in animal societies (Shettleworth, 1998). However,
the knowledge that individuals of one species may have about
species-specific characteristics of heterospecific social companions and how this knowledge is organized in the memory
are poorly understood.
A
Address correspondence to P. Matyjasiak. E-mail: pmatyjasiak@
wp.pl.
Received 18 February 2004; revised 28 September 2004; accepted 26
October 2004.
The ability to recognize and remember species characteristics has long been hypothesized to result from learning by
making associations between multiple traits (e.g., vocalizations, plumage, morphology, and displays) that distinguish
species (Gill and Murray, 1972; Grant BR and Grant PR, 2002;
Grant PR and Grant BR, 1997; Irwin and Price, 1999).
Accordingly, recent studies in birds have shown that species
recognition ability is influenced largely by social learning of
species-specific traits (e.g., Grant PR and Grant BR, 1997;
Hansen and Slagsvold, 2003; Slagsvold and Hansen, 2001),
although there is also a heritable component of the conspecific recognition template (e.g., Hauber et al., 2001; Slagsvold
et al., 2002; Soha and Marler, 2000). However, current
evidence that individuals can associate multiple speciesspecific traits—either increased or decreased vocal discrimination shown by sympatric populations of birds in comparison
to allopatric populations (Catchpole, 1978; Catchpole and
Leisler, 1986; Gil, 1997; Gill and Murray, 1972; Lynch and
Baker, 1990)—is only indirect. Such local differences in vocal
discrimination are suggested to result from ‘‘behavioral experience of the birds’’ and ‘‘learning that a particular song
represents a particular plumage type’’ (Gill and Murray, 1972;
Irwin and Price, 1999). However, they could also originate
from interspecific misimprinting and misidentification (when
‘‘being mistaken’’ and behaving aggressively toward singing
heterospecifics are advantageous; e.g., Emlen et al., 1975; Gil,
1997) or from habituation to heterospecific signal traits
(when ‘‘being mistaken’’ leads to a waste of time and energy
in evicting heterospecific individuals; e.g., Lynch and Baker,
1990). Therefore, associations between multiple signals, even
if suggested, may be both unnecessary and absent. Rigorous
experiments are thus required to evaluate the importance of
associations between different forms of signals in species
recognition.
The objective of this study was to provide a first experimental
test of the hypothesis that associating signal components from
Behavioral Ecology vol. 16 no. 2 International Society for Behavioral Ecology 2004; all rights reserved.
468
Behavioral Ecology
multiple sensory modalities is important for distinguishing
between conspecifics and heterospecifics in a population of
birds. I tested this hypothesis using interspecifically territorial
male blackcaps (Sylvia atricapilla). Male blackcaps defend
breeding territories against both male conspecifics and male
garden warblers (Sylvia borin), which arrive 3–4 weeks later in
spring and resemble the blackcap in song, morphology, and
ecological requirements (Cramp, 1992; Garcia, 1983). The
blackcap and the garden warbler are closely related sister
species (Blondel et al., 1996). The functional significance
of interspecific territoriality is unknown in these species,
however, and there is no evidence that they hybridize in the
wild (Cramp, 1992). Male blackcaps defend their large,
multipurpose territories by song, displays, and chases and
respond in a hostile way to garden warbler song (Cramp,
1992; Garcia, 1983). Here I ask whether species recognition
ability during territory defense in male blackcaps is based on
making links between song and plumage or whether blackcaps’ aggressive response toward male garden warblers is
merely an effect of an inability to distinguish these heterospecific males from conspecific males (Lynch and Baker, 1990;
Murray, 1981). Assuming the former to be true, I tested
whether male blackcaps remember these associations from
year to year. As blackcaps and garden warblers differ in timing
of migration and their winter quarters are geographically
separated (Cramp, 1992), this would require a long-lasting
(cross-season) memory of species-specific features for the
8 months or more that separate spring arrival of male garden
warblers from their departure in the previous autumn. I also
investigated whether such species recognition abilities exist in
yearling male blackcaps. If so, this would mean that these
abilities develop in yearling males outside the context of their
own experience of territory defense, that is, prior to their first
breeding episode.
METHODS
I studied blackcaps and garden warblers in the Kampinoski
National Park (near Warsaw, central Poland; 52 229 N, 20 509
E). Both species are abundant there and co-occur throughout
various moist and wet deciduous forest and scrubland habitats, such as alder and ash-alder swamp forests, hornbeamoak-lime forests, and willow-birch secondary thickets. Blackcaps
arrive at the study area between mid-April and mid-May, and
garden warblers arrive on average after 5 May. Both species
extensively overlap in their nesting temporally and spatially,
and their breeding territories overlap in my study area.
Playback experiment
To determine whether species recognition ability in male
blackcaps is based on making links between song and
plumage, I used dual-choice song playback trials (Figure
1A). The trials were performed on 80 territorial male
blackcaps in a 2-week period from 21 April to 5 May 1996
(prenesting and early nesting stage in the breeding cycle of
the blackcap), preceding the arrival of garden warblers from
their winter quarters. This enabled me to test male blackcaps
that had not had contact with male garden warblers for 8 or
more months. This also allowed me to examine yearling male
blackcaps (in their second calendar year of life) that had
never defended breeding territories against male garden
warblers. Yearlings make up about 60% of males that respond
to playbacks of garden warbler songs in this blackcap
population. This is based on bird captures run in the same
area using song playback (Matyjasiak, unpublished data; n ¼
61 males), during which I separated male blackcaps into
Figure 1
Responses of male blackcaps to garden warbler and blackcap songs
during dual-choice playback experiments. (A) Diagram showing the
experimental setup. Black-capped birds symbolize male blackcaps,
plain birds–male garden warblers, and gray birds—male chaffinches.
Birds atop the figure sections represent intruder-seeking male
blackcaps (territory owners), while stuffed male models (which
simulated intruders) are shown at the sides. The speaker is positioned
in between the male models. (B) Trials in which garden warbler
songs were being played. (C) Trials in which blackcap songs were
being played. Numbers denote the number of male blackcaps that
attacked a particular male model (total n ¼ 80 males).
Matyjasiak
•
Species recognition and association of acoustic and visual cues
yearlings and older males on the basis of their molt patterns
and feather quality (Jenni and Winkler, 1994).
I observed singing male blackcaps and mapped their
territories for 1–2 days preceding the days of playback trials.
Males that responded to garden warbler songs and that
occupied nonneighboring territories (to avoid testing the
same male twice) were chosen for the trials. I identified these
males by their response to garden warbler songs played back
from a portable Realistic tape recorder. (These playback tapes
were not used in trials.) By tossing a coin, I assigned males to
tests with garden warbler or blackcap songs. For each male
blackcap, I placed two stuffed male models within its territory
(mounted on 1.5-m-tall sticks placed 4 m apart), and in
between the male models, I placed a loudspeaker (Tonsil
[Września, Poland], 10 W, mounted on 0.5-m stick) (Figure
1A). The two models, one of a male blackcap and one of
a garden warbler male in full breeding plumages, represented
a paired stimulus. The area ranging up to 5 m from the loudspeaker is hereafter referred to as the ‘‘test arena.’’ The position of models with respect to the loudspeaker (i.e., whether
they were on one side or the other of the loudspeaker as
viewed from the observation point) was randomized. Then, I
played back either garden warbler or blackcap territorial
songs (UHER Report 4000IC tape recorder). Such a playback
test attempted to imitate the territorial intrusion and advertisement of an unmated male. I terminated each playback when the
intruder-seeking territory owner entered the test arena (i.e.,
approached at 5 m or closer to the loudspeaker), and I noted to
which of the two male models it would approach and direct its
aggressive response. By aggressive response I meant displaying
to the model (a male facing the model closely and beating its
wings) or attacking the model (a male touching the model
physically or pecking at it while sitting on its back). These
behaviors are typical of antagonistic interactions between male
blackcaps during territorial disputes (Cramp, 1992). I also
recorded the time that elapsed since the onset of the song
playback to male approach at the test arena (accuracy of 1 s).
In experimental trials with garden warbler songs, male
blackcaps could approach the garden warbler male model not
because they correctly associated species-specific song with
species-specific plumage but because they chose any male
model that was not a male blackcap. To control for this
possibility, I performed control trials. These were of the same
design as the experimental trials except that male blackcaps
were given a choice between two heterospecific male models:
one of a garden warbler and one of a chaffinch (Fringilla
coelebs) male in full breeding plumage.
All trials were performed in the morning (0600 to 1200 h)
in calm weather. Each male blackcap was tested only once. I
examined the following number of male blackcaps in this
playback study (total n ¼ 80 males): 41 males in trials with
garden warbler songs (of which 31 males were tested in
experimental and 10 males in control trials) and 39 males in
experimental tests with blackcap songs. Additionally, I played
back alternately garden warbler and blackcap songs to five
male blackcaps. The purpose of this was to explore whether, if
they did discriminate between the two species, males would
switch responses on a short time scale. I played back garden
warbler and blackcap songs alternately in 5-s bouts interspersed with 10 s of silence. The species-specific songs were
played back twice each in the following order: garden warbler–
blackcap–garden warbler–blackcap. The results of these additional trials were analyzed descriptively.
The song stimuli consisted of songs of blackcap and garden
warbler males, which I recorded from a location situated in
the Kampinoski National Park 5 km from the present study
plot, in 1992. I used multiple song stimuli to represent the two
populations of potential stimuli: eight tapes with blackcap and
469
nine tapes with garden warbler songs (for discussion of
experimental design of playbacks, see Kroodsma, 1989;
McGregor, 1992). Each tape represented a song recording
obtained from a different individual. To prepare a playback
tape, 4 min of continuous natural song from one focal male
was selected at random and recorded onto a playback tape.
Songs were recorded from distances ,10 m, at dawn, using
a UHER M518 omnidirectional microphone installed in a
95-cm parabola and a UHER Report 4000IC tape recorder
(recording at 19 cm/s). The selection of playback tapes was
randomized.
I compared proportions of male blackcaps that approached
the male models using chi-square tests and the latencies to
enter the test arena using the Mann-Whitney U test. If male
blackcaps can associate species-specific songs with speciesspecific plumage, they should approach, display to, or attack
male models of the warbler species, the song of which had
been played earlier during the playback trial.
RESULTS
Male blackcaps preferentially behaved aggressively against
male models of the warbler species whose song had been
played earlier during the playback trial. In experimental trials
with garden warbler songs, males behaved aggressively against
the garden warbler male model significantly more often than
against the blackcap male model (n ¼ 31 males, v2 ¼ 17.06,
df ¼ 1, p , .001) (Figure 1B). In control tests with garden
warbler songs, males behaved aggressively against the garden
warbler male model but not against the control chaffinch
male model (n ¼ 10 males, v2 ¼ 10.0, df ¼ 1, p , .002)
(Figure 1B). In tests with blackcap songs, all male blackcaps
behaved aggressively against the blackcap male model and
none against the garden warbler male model (n ¼ 39 males,
v2 ¼ 39.0, df ¼ 1, p , .001) (Figure 1C). Numbers of males
that ended up being tested with each tape were as follows (in
parentheses are given numbers of males that incorrectly
identified the species in garden warbler song trials): experimental trials with garden warbler songs (nine tapes)—4 (1),
4 (1), 4 (1), 3 (0), 4 (0), 3 (1), 3 (0), 3 (0), 3 (0); control trials
with garden warbler songs (nine tapes)—2, 2, 1, 1, 1, 1, 1, 0, 1;
and experimental trials with blackcap songs (eight tapes)—5,
4, 5, 5, 6, 4, 5, 5. The five male blackcaps that I tested with
alternating playback of conspecific and heterospecific song
switched between behaving aggressively against blackcap and
against garden warbler models, consistent with the species
whose song was being played. This result indicates the
repeatability of male behavior during playback experiments.
There was no difference in approach latency between
experimental tests with garden warbler songs and experimental tests with blackcap songs; in both cases, male blackcaps
entered the test arena in a median time of 35 s (Mann-Whitney
U test: z ¼ 0.28, p . .70, n ¼ 70 males tested) (Figure 2).
DISCUSSION
In this study, I tested the hypothesis that birds are able to
associate species-specific songs with species-specific plumage
types. The results show that male blackcaps use both vocal and
visual stimuli for species recognition and that they have
knowledge of which species-specific songs represent which
species-specific plumage. Median times taken for male blackcaps to identify and respond to singing conspecific and
heterospecific simulated intruders were similar. The five males
that were tested with alternating playbacks of conspecific and
heterospecific song were able to switch their antagonistic
response over a short time scale. These results suggest that the
blackcaps’ aggressive response to garden warbler song is not
Behavioral Ecology
470
Figure 2
Median time interval (approach latency) between the onset of
playbacks of blackcap and garden warbler songs and the appearance
of male blackcaps at the test arena, that is at 5 m or closer to the
playback speaker. White boxes ¼ quartiles, error bars ¼ the range.
Numbers of male blackcaps tested are shown in parentheses (total
n ¼ 70 males).
a mere effect of an inability to distinguish the species’ songs.
This eliminates misdirected intraspecific aggression (Murray,
1981) as the proximate explanation for the aggressive response to the garden warbler song of male blackcaps.
The results presented here are consistent with the
hypothesis that distinguishing between conspecifics and
heterospecifics is based on associating signal components
from multiple sensory modalities (here visual and auditory)
(Gill and Murray, 1972; Irwin and Price, 1999). Furthermore,
male blackcaps could not only associate garden warbler songs
with garden warbler plumage but they were also able to
remember and recall this association after 8 months without
contact with garden warblers. I propose that this ability helps
males remember opponents and defend the territory. It may
allow birds to avoid wasting time and energy spent evicting
heterospecific intruders in ecological conditions in which
they pose no threat. Presumably, birds acquire networks of
associations among species-specific features of conspecific and
heterospecific social companions that guide their behavior
during social interactions. Long-lasting memory of these
associations may represent an evolutionary adaptation in
cognitive abilities in populations of migratory birds, like the
blackcap. The ability to remember associations between
multimodal signals may be advantageous also in animals
that assess their sexual competitors or choose a mate using
multiple sexual ornaments, like song quality and plumage
brightness. It should also exist in brood-parasitic bird species.
The age of tested males could not be estimated in the
present study, but males were chosen randomly for dualchoice trials from the population of male blackcaps that
respond to playbacks of garden warbler songs. The proportion
of males that incorrectly identified the species in garden
warbler song trials (13% among 31 males that were tested with
garden warbler songs) was significantly lower (v2 ¼ 25.9, df ¼
1, p , .001) than the proportion of yearling males in this
blackcap population (60% among 61 males that responded to
playbacks of garden warbler songs; Matyjasiak, unpublished
data). The average life expectancy of male and female blackcaps is about 2 years (Berthold et al., 1990); hence, male
blackcaps can usually enjoy only one to two breeding seasons
in their life. Therefore, the results indicate that the ability to
identify heterospecific competitors must develop in these
short-lived birds before the first breeding attempt. Yearling
male blackcaps presumably develop the mental representation of male garden warblers the preceding summer. This
implies that species recognition skills learned in one context
can be used in another context: male blackcaps learn, in the
year of their birth, to link song and plumage for species
recognition several months prior to using this ability while
defending their territories during their first breeding season
in the following year. This mechanism is likely to apply in the
case of conspecific territorial behavior too.
This is the first experimental evidence, to my knowledge,
that associating signal components from different sensory
domains is important for species recognition in animals. I
suggest that, in the case of ecological competitors, the
development of such associations could particularly be
expected in animals living in cluttered habitats or in crowded
social environments. Individuals could use one signal trait
(e.g., song that identifies the species at a long range) to
identify individuals that differ in other signal traits (e.g., those
used in short-range species identification, like displays, morphology, and plumage). Future study is needed to establish
whether individuals can utilize such a species identification
ability to adjust their aggressive response toward heterospecific competitors according to changing ecological conditions
(e.g., availability of food and potential nest sites or intruder
pressure).
I thank A. Wasilewski for supervising and support; J. Misiak, the
director of the Kampinoski National Park, for permission to access the
park area; P. Koz1owski, the curator of the Natural History Museum
(Institute of Zoology PAS), for allowing to use stuffed birds in my
experiments; A.F.G. Bourke, E. Danchin, P.G. Jab1oński, A.P. Møller,
and two anonymous referees for constructive comments on the manuscript. The experiment conducted in this study complies with current
animal welfare laws of Poland. I was supported by a grant from the
State Committee for Scientific Research (KBN) grant 6P04F 03009
and by a postdoctoral fellowship from the Foundation for Polish
Science (FNP).
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