Behavioral Ecology
doi:10.1093/beheco/arj052
Advance Access publication 22 February 2006
Song discrimination suggests premating
isolation among sympatric indigobird
species and host races
Christopher N. Balakrishnan and Michael D. Sorenson
Boston University, Department of Biology, Boston, MA, USA
Indigobirds (Vidua spp.) are host-specific brood parasites that have diversified in a recent radiation apparently driven by host
colonization. Behavioral imprinting of both male and female indigobirds on host song is thought to promote rapid speciation
because it results in assortative mating between indigobirds associated with a particular host. We conducted a song playback
experiment to test whether male indigobirds discriminate among potential competitors based on song. Of particular interest was
the behavior of two sympatric host races of the Cameroon indigobird Vidua camerunensis that differ only in host song mimicry and
other components of their vocal repertoires. Territorial males of the two V. camerunensis host races and Vidua raricola, a morphologically distinct indigobird species, were tested with playbacks of each other’s songs. Males of all three groups responded most
aggressively to songs of their own species and/or host race, as evidenced by strong and highly significant differences in a variety of
response variables. This differential territorial defense suggests that an intruding male with different songs does not represent
a competitive threat and is expected if females mate assortatively with respect to song. Thus, our results provide evidence of
premating reproductive isolation among recently evolved indigobird species and host races. Key words: brood parasitism, indigobird, playback experiment, reproductive isolation, song, speciation. [Behav Ecol 17:473–478 (2006)]
ecent studies of speciation reflect a shift in emphasis from
the geographic context of speciation to the processes promoting population differentiation (Kirkpatrick and Ravigné,
2002; Schluter, 1996, 1998). One such process is the divergence of mate recognition signals. While the evolution of
postmating reproductive isolation between bird species can
be a relatively protracted process (Prager and Wilson, 1975;
Price and Bouvier, 2002), premating isolation may result from
the relatively rapid divergence of sexually selected traits such
as song (Price, 1998). Among birds, song divergence may
therefore be of particular importance in the early stages of
the speciation process (Edwards et al., 2005; Slabbekoorn and
Smith, 2002a).
Ten species of African indigobirds (Vidua spp.) are hostspecific brood parasites of various estrildid finches (Payne,
2004). Both behavioral and genetic data suggest that sympatric speciation via host shift has played an important role in the
diversification of indigobird species (Klein and Payne, 1998;
Payne et al., 1998, 2000; Sorenson et al., 2003). Host colonization is facilitated by behavioral imprinting of indigobirds on
the particular host species that raises them. Male indigobirds
imprint on the songs of their foster parents and include mimicry of host song in a repertoire that also includes vocalizations unique to indigobirds (Payne et al. 1998). Though they
do not sing, female indigobirds also imprint on their host
species and mate preferentially with indigobird males that
mimic the songs of the host that reared them (Payne et al.,
2000). After the colonization of a new host, social imprinting
results in simultaneous divergence in male courtship songs
and female preferences for male songs, as well as female preferences for host nests (Payne et al., 2000), and therefore leads
to reproductive isolation.
R
Address correspondence to C.N. Balakrishnan. E-mail: cbala@oeb.
harvard.edu.
Received 29 August 2005; revised 15 January 2006; accepted 17
January 2006.
The Author 2006. Published by Oxford University Press on behalf of
the International Society for Behavioral Ecology. All rights reserved.
For permissions, please e-mail: [email protected]
The first step in speciation by host shift is the formation of
conspecific host races (Bush, 1969). Although most indigobird species are associated with a single estrildid finch host,
the Cameroon indigobird Vidua camerunensis is associated
with several host species across West Africa (Payne et al.,
2005). Indigobirds associated with these hosts have been
treated as a single species because they are essentially indistinguishable in both morphology (Payne et al., 2005) and
neutral genetic markers (Sefc et al., 2005; Balakrishnan CN
and Sorenson MD, in preparation). Near Tibati, Cameroon,
and elsewhere in West Africa V. camerunensis males mimic the
songs of either African firefinch Lagonosticta rubricata or blackbellied firefinch Lagonosticta rara. Given social imprinting
on hosts, these birds may represent two reproductively
isolated host races that have not yet evolved morphological
differences after recent colonization of a new host (Payne
et al., 2005).
As a test of reproductive isolation between indigobird species and host races, we conducted a song playback experiment
to determine whether male indigobirds discriminate among
potential competitors based on song. This experiment follows
the logic of previous avian studies in which song playbacks
to males have been used to test for premating reproductive
isolation (e.g., Grant BR and Grant PR, 2002a,b; Irwin et al.,
2001; Patten et al., 2004; Salomon, 1989). If female indigobirds mate exclusively with males mimicking the appropriate
host song, then a male indigobird mimicking the song of
a different host does not represent a competitive threat to
a territorial male and should not elicit an aggressive response.
Alternatively, if females mate with males irrespective of their
host association (as advertised by mimicry song), territorial
males should be aggressive toward all male indigobirds.
Therefore, if males behave adaptively in response to potential
competitors, song discrimination by males provides indirect
evidence of premating reproductive isolation based on song.
We tested male territorial responses in two host races of
V. camerunensis and a second species, the Jambandu indigobird
Vidua raricola.
Behavioral Ecology
474
Figure 1
Representative audiospectrograms of the three general classes of songs (chatter, complex nonmimicry, and mimicry) for song populations of
indigobirds. (A) Vidua camerunensis: Lagonosticta rara mimics, (B) V. camerunensis: Lagonosticta rubricata mimics, and (C) Vidua raricola. Note that
song repertoires of individual males include many distinct nonmimicry songs (each comprised of a complex series of notes) and several types of
mimetic vocalizations. See Payne RB, and Payne LL (1994) and Payne et al. (2005) for side-by-side comparisons of mimetic indigobird songs and
host songs.
Playback stimuli
MATERIALS AND METHODS
Locality and natural history
2
Fieldwork was conducted in an approximately 10-km area
north and west of Tibati, Cameroon (628# N, 1334# E) from
October through December of 2001 through 2003. At Tibati,
three indigobird species are associated with four hosts. Individual V. camerunensis males representing two putative host
races mimic the songs of either L. rubricata or L. rara (Payne
et al. 2005). In contrast, V. raricola, which parasitizes zebra
waxbill Amandava subflava, is morphologically distinct and
significantly differentiated from V. camerunensis in neutral
genetic markers (Sefc et al., 2005; Balakrishnan CN and
Sorenson MD, in preparation). A third species that is less
numerous at Tibati, Wilson’s indigobird Vidua wilsoni, was
not included in this study. V. camerunensis and V. raricola are
distributed syntopically in small agricultural fields and surrounding bush, with individuals of different species or host
races sometimes defending adjacent territories.
Male indigobirds are highly territorial during the breeding
season, and song playbacks elicit aggressive behavior (e.g.,
Payne, 1973). Territorial males sing conspicuously from a habitual ‘‘call site’’ at the top of a tree. Female indigobirds visit
male call sites in order to choose a mate and typically copulate
at the call site. The indigobird social system has been described as a ‘‘dispersed lek,’’ and male reproductive success
appears to be highly skewed (Payne RB and Payne K, 1977).
Thus, song plays a fundamental role in the indigobird mating
system. Based on male singing and territorial behavior, indigobird species show substantial overlap in the timing of their
breeding seasons. To simplify description of the results, each
of the two V. camerunensis host races and V. raricola will be
referred to as a ‘‘song population,’’ and playback songs will
be described as either homotypic or heterotypic with respect
to host song mimicry.
Indigobirds were recorded with a Sony TCDM5 cassette recorder and a Sennheiser ME66 shotgun microphone. During
2001, we recorded more than 100 males on their call sites,
generally collecting about 15 min of song data per individual.
Recordings were transferred to minidisc for long-term storage
after fieldwork each day using a Sony MZ-R700 minidisc recorder. Audiospectrographs were created and analyzed using
AviSoft SASLab Pro version 4.3 (AviSoft Bioacoustics, Berlin,
Germany). Playback tapes were made from a subset of the
recordings made in 2001. Indigobird vocalizations are remarkably complex, incorporating mimicry of host songs, ‘‘chatter’’
calls, and numerous nonmimicry songs, each comprising a
unique sequence of notes (Figure 1). Chatter calls are similar across species, whereas both mimicry and nonmimicry
songs vary between species (Payne, 1973, 1982) and host races
(Balakrishnan CN and Sorenson MD, unpublished data).
Young males apparently learn nonmimicry songs from adult
male indigobirds that mimic the same host species. Clear recordings (high signal to noise ratio) containing chatter and
both mimetic and nonmimetic song components were selected for use in the playback experiment. Each tape consisted
of 5 min of continuous, unaltered courtship song from a different male. Nine unique playback tapes were prepared for
each of the three song populations.
Playback experiments
Playback experiments were conducted during the breeding
season from October to December of 2002 (n ¼ 72 trials) and
2003 (n ¼ 9 trials). We tested male indigobirds because their
territorial responses could be readily quantified in the field and
because their vocalizations provide unambiguous evidence of
host association. In contrast, females of the study species are
Balakrishnan and Sorenson • Premating isolation among indigobirds
Table 1
Design of playback experiment for each study population
Focal bird
Trial No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 No. 7 No. 8 No. 9
1
2
3
A1
B1
C1
B2
C2
A2
C3
A3
B3
A4
B4
C4
B5
C5
A5
C6
A6
B6
A7
B7
C7
B8
C8
A8
C9
A9
B9
Each of nine focal birds per song population was tested with a unique
set of three recordings, with the order of song types systematically
shuffled among males. A1–A9 (nine unique recordings of Vidua
camerunensis, L. rara mimics), B1–B9 (nine unique recordings of
V. camerunensis, Lagonosticta rubricata mimics) and C1–C9 (nine
unique recordings of Vidua raricola). The same set of 27 recordings
was used for playbacks to each of the three song populations.
morphologically indistinguishable and do not vocalize, making
it impossible to determine host association or species identity.
Playbacks were broadcast at natural volume, approximating
that of the focal bird, using a Sony TCDM5 tape deck and
a Sony SRS-A27 speaker placed on or near the ground at
a distance of 15–20 m from the call site of the territorial male.
In order to provide a focal point for aggressive responses,
a taxidermic mount of a male indigobird was attached to
a branch near the speaker at a height of approximately 1 m.
To control for the possibility of differential responses based
on morphology of the intruding male, a specimen of Vidua
purpurascens was used in all experiments; V. purpurascens does
not occur in West Africa and is morphologically distinct from
both of the focal species in this study. The speaker and taxidermic mount were placed in a location that allowed focal
birds to perch in close proximity.
Nine males of each song population were challenged with
recordings of other males from each of the three song populations (total: 27 males and 81 trials). To avoid pseudoreplication (Kroodsma, 1989), each individual within a song
population was presented with a unique set of three recordings, the order of which was shuffled systematically to control
for possible habituation across trials (Table 1). All playbacks
were conducted between 0700 and 1100 h, and successive trials
involving a given individual were separated by a minimum of 2
days. All males tested were present at their call sites for at least
2 days prior to the first playback trial. Studies of banded indigobirds show that individual males generally occupy a single
call site throughout the breeding season and sometimes return
to the same call site the next year (e.g., Payne, 1973).
Each trial included a 5 min period prior to playback during
which the focal male was recorded, 5 min of song playback,
and a 5 min post-playback recording period. During playback,
behavioral responses of the focal male were noted by dictation
onto a minidisc recorder. Response measures included: time of
first response (latency), duration of response (duration), number of flights toward the speaker (flights), number of hops
toward the speaker (hops), amount of time spent within 1 m
of the speaker (close), amount of time spent out of sight (absent), and the number of mimicry songs performed by the
focal male (mimicry songs). During the first 10 trials, we also
included a 5 min period prior to preplayback recording during
which the taxidermic mount was presented without any accompanying song. No birds showed any response to the mount
alone, so this procedure was omitted in subsequent trials.
Statistical analysis
Statistical analyses were conducted in SYSTAT, version 5.2
(Wilkinson, 1992). All response measures were log (x 1 1)
475
transformed because of high variance and skew; many of the
response measures had values of zero for multiple trials. Because response measures were correlated, we used principal
components analysis (PCA) to reduce the data to a set of uncorrelated response variables. Principal components were
used as response measures in a nested analysis of variance
(ANOVA) design in which individual focal birds were nested
within song populations. Trial number (1, 2, or 3 for each
focal bird) and experiment date were included as covariates
in the ANOVA to test for possible effects of habituation to playbacks and seasonal changes in responsiveness. Pairwise, post
hoc comparisons were made using Fisher’s least significant
difference test with correction of significance values for multiple comparisons (Benjamini and Hochberg, 1995).
RESULTS
Responses to homotypic versus heterotypic songs
Responses to playback of homotypic song were clearly stronger than responses to heterotypic song. Males responding to
homotypic song typically made repeated flights back and forth
over the mount or a slow, gradual approach toward the
mount. In contrast, heterotypic song elicited either no response or a rapid, but short-lived approach that was followed
by the resumption of normal courtship singing at a distance
from the playback speaker and taxidermic mount. Consistent
differences were observed in all seven response measures, with
birds of each song population responding more strongly to
homotypic than heterotypic song (Table 2). The first two axes
in the PCA explained 72% of the variation in the data. The
first principal component (PC1) explained 54% of the variation and can be interpreted as an index of aggression, with
positive loadings for duration (0.87), close (0.75), flights
(0.89), and hops (0.83) and negative loadings for latency
(0.71), absent (0.51), and mimicry song (0.46).
A nested ANOVA using PC1 as the response measure indicated a highly significant interaction between focal bird song
population and playback song (F4,45 ¼ 17.29, p , 105).
Specifically, male territorial responses were strongest when
the simulated intruder was from the same song population
as the focal male. Pairwise tests show that each of the three
indigobird song populations responded more strongly when
presented with homotypic as compared to either of the two
heterotypic songs (Figure 2). Thus, the two V. camerunensis
host races responded no more aggressively to each other than
to heterospecific recordings of V. raricola. Individual birds differed significantly in aggressiveness (F24,45 ¼ 1.97, p , .05) but
there were no significant effects of habituation (trial number)
or experiment date on aggressive responses.
PC2 had a strongly positive loading for mimicry song (0.72)
and a strongly negative loading for absent (0.74), simply
reflecting a negative correlation between the number of mimicry songs delivered by focal males and the time they were away
from the call site during the playback trial. However, neither
this response measure was significantly related to any of the
independent variables or covariates in the nested ANOVA analysis nor did the interaction of focal male song population and
playback song significantly affect magnitude of PC2. Likewise,
no significant results were obtained for PC3, which explained
only 9.7% of the variation in the data (eigenvalue ¼ 0.68).
Focal male vocalizations during and after playback trials
Further examination of the singing behavior of focal males
suggests different responses to playback of homotypic versus
heterotypic song. When challenged with heterotypic songs,
indigobirds generally continued singing their normal, mixed
Behavioral Ecology
476
Table 2
Untransformed means of response measures for each of the three focal bird song types when presented
with each of three different stimulus songs
Playback type
Focal species
Response variable
Lagonosticta rara
mimic
Vidua camerunensis: L. rara mimics (n ¼ 9)
Latency
15.55 (13.92)
Duration
272.67 (51.18)
Close
76.00 (88.34)
Flights
9.67 (6.34)
Hops
4.11 (3.82)
Absent
10.89 (21.70)
Mimicry song
0.11 (0.33)
V. camerunensis: L. rubricata mimics (n ¼ 9)
Latency
158.11 (131.11)
Duration
88.78 (94.51)
Close
3.11 (9.33)
Flights
1.78 (1.86)
Hops
1.67 (2.55)
Absent
45.89 (58.17)
Mimicry song
2.00 (2.55)
V. raricola (n ¼ 9)
Latency
72.44 (99.81)
Duration
83.89 (99.09)
Close
3.56 (6.00)
Flights
2.22 (3.23)
Hops
1.33 (1.50)
Absent
93.67 (108.82)
Mimicry song
1.67 (2.60)
Lagonosticta rubricata
mimic
Vidua raricola
121.11
50.89
0.22
2.00
0.88
80.78
3.33
(134.36)
(65.79)
(0.67)
(2.12)
(1.26)
(96.21)
(4.24)
145.00
69.78
0.33
0.88
1.22
98.44
1.89
(136.18)
(100.91)
(1.00)
(1.17)
(1.64)
(72.72)
(3.89)
65.66
189.11
10.22
5.44
5.44
35.00
0.33
(97.28)
(104.92)
(24.53)
(5.17)
(5.17)
(69.73)
(0.50)
115.78
104.94
1.67
1.44
1.67
28.33
2.11
(114.67)
(85.00)
(5.00)
(2.01)
(1.66)
(76.91)
(4.17)
90.89
105.67
5.33
3.88
1.33
117.89
0.67
(122.45)
(100.56)
(10.59)
(4.99)
(1.58)
(84.81)
(1.11)
36.56
240.22
42.89
9.11
4.44
7.88
0.67
(69.07)
(91.87)
(32.45)
(3.66)
(3.88)
(17.69)
(1.66)
Responses to homotypic playbacks are shown in bold. Standard deviations are given in parentheses.
Latency, duration, close, and absent are measured in seconds; the remaining response measures are
measured as simple counts.
repertoire. In contrast, when challenged with homotypic songs,
indigobirds often became silent or primarily gave chatter calls.
A reduction in mimicry song in aggressive situations (i.e., homotypic playbacks) was suggested by the negative loading of
mimicry song in PC1 (0.46). An ANOVA using the nested
design described above and mimicry song as a univariate response measure indicated a nonsignificant interaction between
playback song and focal bird song population (F4,52 ¼ 2.12,
Figure 2
Average PC1 scores (6SE) for
each of the three focal male
song types when presented
with each of three different
types stimulus songs. (A) Playback of Lagonosticta rara mimics,
(B) playback of Lagonosticta
rubricata mimics, and (C) playback of Vidua raricola (mimics
Amandava subflava). Each mean
is based on nine trials. All pairwise comparisons between homotypic and heterotypic playbacks
were significant by Fisher’s least
significant difference test (*p ,
.05, **p , .01, ***p , .001).
p ¼ .09), but with a trend toward less mimicry song in response
to homotypic playbacks. A stronger negative relationship was
evident in a partial regression of aggressive response (as measured by PC1 for the six nonvocal response variables) and the
number of mimicry songs (F1,50 ¼ 12.06, p ¼ .001). Thus, those
indigobirds responding most aggressively sang less mimicry
song during playback trials. Counts of mimicry and nonmimicry vocalizations given by focal males revealed no apparent
Balakrishnan and Sorenson • Premating isolation among indigobirds
477
differences in vocal repertoire between the 5 min periods before and after playback trials, indicating that the playback trials
had no lasting effects on the singing behavior of focal males.
transient stage would undo behaviors that have been advantageous over the longer term of indigobird evolution. Finally,
the dispersed lek mating system of indigobirds, combined
with choice based on signals that can be detected at a distance, likely minimizes the costs of female choice in this
system.
Clearly, playback experiments with females would allow
a more direct test of reproductive isolation, but are logistically
difficult because female indigobirds do not respond well to
playbacks in the field (e.g., Payne, 1973). In white-crowned
sparrows, females appear to be less discriminating than males
in their responses to local and foreign dialects (Nelson and
Soha, 2004), whereas the consequences of song recognition
learning are similar in male and female zebra finches (Riebel
et al., 2002). Other studies comparing male and female responses are difficult to evaluate because experimental procedures for males and females often are necessarily different
(Ratcliffe and Otter, 1996; Reibel et al., 2002). In addition,
males and females may respond to different components of
the song repertoire.
Consistent with this possibility, the changes we observed
in song repertoire by focal birds during playback of homotypic song suggest different functions for different components of indigobird song. Payne (1979) found correlations
between song type (chatter, complex nonmimicry, and mimicry) and behavioral context. In particular, chatter calls were
delivered more often in aggressive contexts, whereas mimicry
song was used more often in sexual contexts. The results presented here provide corroborating experimental evidence
that mimicry songs function primarily in courtship and are
rarely delivered in aggressive contexts. Chatter vocalizations,
which are similar in tonal quality and note structure across
song populations (Figure 1), may elicit initially aggressive responses even in heterotypic playbacks and may explain observations of occasional interspecific aggression in indigobirds
(e.g., Payne and Gropschupf, 1984). After an initial response,
males may modify their behavior once an intruding male
has been identified by its mimicry and/or complex nonmimicry songs.
We used recordings of natural song sequences including
both mimetic and nonmimetic songs to simulate territorial
intrusions by males of different song populations and therefore cannot assess the relative contributions of these song
components in eliciting male territorial responses. In a previous study, male Vidua chalybeata and V. purpurascens in southern Africa responded more strongly to conspecific than
heterospecific nonmimicry songs, but were generally unresponsive to mimicry songs when each was played back separately (Payne, 1973). In contrast, playbacks to a sample of four
captive females indicated a preference for conspecific mimicry song, but no preference for nonmimicry song (Payne,
1973), suggesting that male and female indigobirds might respond to different attributes of song. A lack of response to
individual song components, however, does not necessarily
mean that they are not important when delivered in the context of a natural mixed repertoire. For example, the mimetic
songs of intruding males, when delivered along with chatter
and nonmimicry vocalizations, may contribute to discrimination by territorial males.
While the available evidence indicates that both male and
female indigobirds respond more strongly to conspecific song
(Payne, 1973; Payne et al., 2000; this study), further experiments using artificial combinations of mimicry and nonmimicry songs from different song populations are needed to assess
the relative roles of these vocalizations in eliciting both male
territorial responses and female mate preferences. Responses
to the full courtship repertoire, however, are perhaps most
relevant to tests of reproductive isolation. Our study supports
DISCUSSION
Male indigobirds showed dramatically different responses to
experimental playback of homotypic versus heterotypic song,
indicating an ability to recognize and discriminate against
potential competitors even in the absence of other phenotypic
cues. If these differential territorial responses are adaptive,
our results suggest that male indigobirds singing heterotypic
song are not significant sexual competitors and, in turn, that
females are discriminating in their choice of mates, preferring
males mimicking the songs of their particular host. Indiscriminate mating by females would select for generalized territorial defense against any intruding male indigobird. Previous
studies have used playbacks to males and similar logic to test
for evidence of reproductive isolation (e.g., Grant BR and
Grant PR, 2002a,b; Irwin et al., 2001; Patten et al., 2004;
Salomon, 1989).
Rapid divergence of sexual signals after the colonization of
a new niche has been observed in other taxa (Danley and
Kocher, 2001; Patten et al., 2004; Slabbekoorn and Smith,
2002b) and may be of general importance in speciation
(Beltman et al., 2004). This process is particularly well understood in indigobirds in which the acquisition of new songs and
mating preferences is an essentially automatic consequence
of host colonization (Payne et al., 1998, 2000, 2002; see also
Beltman and Haccou 2005; Beltman et al., 2004). Our results
suggest that two morphologically and genetically indistinguishable populations of V. camerunensis represent sympatric
host races isolated by song, perhaps exemplifying the early
stages of speciation by host shift. Even if females mate exclusively with males of the same song type, however, imperfect
specificity in female host choice (i.e., where they lay eggs)
could lead to continuing gene flow between host races (see
Payne and Sorenson, 2004); but this would not select for generalized territoriality among males.
While consistent with the overall model of host imprinting
determining both male sexual signals (song) and female
mate preferences, our results allow only an indirect inference
about female behavior. If mate choice is costly to females
(e.g., Byers et al., 2005) and there is no particular advantage
to mating with a male of the same song type, selection on
females might favor indiscriminate mating. In general, however, mate choice based on song is probably strongly favored
in indigobirds because it produces offspring that are well
adapted to their hosts. Young indigobirds mimic the hostspecific mouth markings of host young (Payne, 2004, 2005)
and male song mimicry, by advertising success in the nest of
a particular host species, serves as a signal of mouth marking
genes appropriate to that host (Payne et al., 1998). Just after
the colonization of a new host, however, this relationship
would not hold, such that selection on female choosiness
might be relaxed or reversed. The expression of normal parenting strategies by the new host would likely result in immediate selection on indigobird mouth markings even in the
absence of specific antiparasite adaptations (see Schuetz
2005 a,b), and as mouth markings evolved to match those
of a new host, selection would again favor mate choice based
on song. Throughout this process, however, selection to
avoid mating with other sympatric indigobirds with divergent
mouth markings (V. raricola and V. wilsoni at our study area)
should continue. We do not know if the two V. camerunensis
host races in Cameroon differ in mouth markings, but it is
unlikely that relaxed selection on female choice during this
478
the principal role of song in indigobird speciation by providing clear experimental evidence that male indigobirds discriminate among potential competitors based on song and
extending this result to the case of morphologically and
genetically indistinguishable host races in the wild. Only if
females respond to song differently than males and the lack
of male response to heterotypic intruders is maladaptive
would our results be consistent with an alternative conclusion of interbreeding between individuals of different song
populations.
We thank P. Campbell, T. Kunz, T. Lenz, I. Lovette, R. Payne,
C. Schneider, K. Sefc, A. Shedlock, F. Wasserman, M. Hauber and three
anonymous reviewers for comments on this manuscript. R. Payne provided advice on fieldwork and help with identification of host song
mimicry. K. Njabo, J. Dongmo, and J. Cooper provided assistance in
the field. We thank R. Fotso for helping us initiate our research in
Cameroon. Permits for work in Cameroon were kindly provided by
the Institute of Agricultural Research for Development and the Ministry of the Environment and Forests. This paper is based on work
supported by the National Science Foundation under Grant Nos.
0089757 and 0309249.
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