EUROPEAN JOURNAL OF ECOLOGY
EJE 2016, 2(1): 53-59, doi: 10.1515/eje-2016-0006

Sympatric song variant in mountain
chickadees Poecile gambeli does not
reduce aggression from black-capped
chickadees Poecile atricapillus
Cara L. Snell1, Stefanie E. LaZerte2, 3, Matthew W. Reudink1, Ken A. Otter3

cal Sciences, Thompson
Rivers University, Kamloops, BC, Canada.
Corresponding author,
E-mail: mreudink@tru.ca
2Department of Geogra-

phy and Environmental
Studies, Thompson Rivers
University, Kamloops,
BC, Canada
3

Natural Resources and
Environmental Studies,
University of Northern
British Columbia, Prince
George, BC, Canada.

ABSTRACT

When habitats overlap and species compete for resources, negative interactions frequently occur. Character
displacement in the form of behavioural, social or morphological divergences between closely related species
can act to reduce negative interactions and often arise in regions of geographic overlap. Mountain chickadees
Poecile gambeli have an altered song structure in regions of geographic overlap with the behaviourally dominant black-capped chickadee Poecile atricapillus. Similar to European and Asian tits, altered song in mountain
chickadees may decrease aggression from black-capped chickadees. To test this hypothesis, we conducted
a playback study in Prince George, BC, Canada, to examine how black-capped chickadees responded to the
songs of mountain chickadees recorded in regions where the two species were either sympatric or allopatric.
We used principal component analysis (PCA) to collapse behavioural response variables into a single ‘approach’
variable and a single ‘vocalisation’ variable. We then used mixed-model analysis to determine whether there
was a difference in approach or vocalisation response to the two types of mountain chickadee songs (allopatric
songs and variant sympatric songs). Black-capped chickadees responded with equal intensity to both types of
mountain chickadee songs, suggesting that the variant mountain chickadee songs from regions of sympatry with
black-capped chickadees do not reduce heterospecific aggression. To our knowledge, this is the only instance
of a character shift unassociated with reduced aggression in the family Paridae and raises interesting questions
about the selective pressures leading to the evolution of this song divergence.
KEYWORDS
Poecile atricapillus, Poecile gambeli, chickadee, song, playback, aggression
© 2016 Cara L. Snell et al.
This is an open access article distributed under the Creative Commons Attribution-NonCommercial-NoDerivs license

INTRODUCTION
In birds, studies on closely related species pairs, such
as great tits Parus major versus Eurasian blue tits Cyanistes
caeruleus, mountain chickadees Poecile gambeli versus blackcapped chickadees Poecile atricapillus and varied tits Poecile
varius versus Japanese tits Parus minor, reveal that when these
species pairs live in sympatry, the subordinate species diverges in expression of shared traits from the dominant species,
undergoing a character displacement (Doutrelant et al. 2000;
Grava et al. 2013a; Hamao et al. 2015). For example, Eurasian
blue tits that inhabit regions where few or no dominant great
tits are present produce songs that are similar to those of great
tits (Doutrelant et al. 2000). In regions of sympatry, however,
Eurasian blue tits, which are the subordinate species, add a trill
to the end of their song. Great tits react less aggressively to
the altered Eurasian blue tit trilled songs than they do to either
untrilled Eurasian blue tit song or the song of conspecific great
tits; the character shift in Eurasian blue tit song acts to reduce

When species compete for resources, negative interactions
frequently occur. This is particularly true amongst closely related species, which often inhabit similar ecological niches and
have similar life history traits, and can lead to high levels of
interspecific competition (Dhondt 2012). Character displacement – divergences in ecological, behavioural, morphological
or physiological traits between closely related species in regions of geographic overlap – can act to differentiate closely
related competitors and reduce interspecific competition
(Brown & Wilson 1956). Character displacement has been observed across a range of taxa including several species in the
order Carnivora (Davies et al. 2007) as well as salamanders (Adams 2004), lizards (Huey 1974; Melville 2002), fish (Schluter &
McPhail 1992) and birds (Grant 1972; Doutrelant et al. 2000;
Grava et al. 2013a; Hamao et al. 2015).

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1Department of Biologi-

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dee songs recorded in areas of sympatry (sympatric songs)
than to allopatric mountain chickadee songs (allopatric song
variants).

aggression from great tits (Doutrelant et al. 2000). A study by
Hamao et al. (2015) on Japanese tits and varied tits also revealed a similar pattern. In regions where the two species live
in sympatry, subordinate Japanese tits sing at a lower frequency, acoustically diverging from the songs of varied tits as well as
from the songs of Japanese tits living in regions where varied
tits are not present. These results suggest that the subordinate
Japanese tit alters its song characteristics to avoid harassment
by the dominant varied tit (Hamao et al. 2015); however, playback studies are needed to confirm that this results in reduced
aggression.
In North America, the black-capped chickadee is socially dominant to the mountain chickadee (Grava et al. 2012a;
Grava et al. 2012b; Grava et al. 2013a). Typically, the two species segregate because of different preferences in habitat and
elevation, but historically zones of overlap exist throughout
their ranges. With the onset of forestry practices that create
abrupt transitions between remnant coniferous forests and
second-growth deciduous forest, the potential for habitat-related isolation has decreased (Grava et al. 2012a).
The song of mountain chickadee is similar to that of
black-capped chickadees in containing whistled notes, typically
starting with a single high note followed by one to two lower
notes (Lohr 2008, Grava et al. 2013a). Whilst the internal syntax, number of notes and frequency ratios within and between
notes of black-capped chickadees are highly stereotyped
across their distributional range (see review in Mennill and
Otter 2007), mountain chickadees exhibit greater range-wide
variation in number of notes and frequency changes between
notes in their song than do the black-capped chickadees (Lohr
2008). This variation may be driven by dominance-mediated
character displacement. Grava et al. (2013a) found differences
in mountain chickadee song structure between regions of cooccurrence and isolation, whereas black-capped chickadees
have a consistent song throughout North America. In areas of
contact between mountain chickadees and black-capped chickadees, the mountain chickadee song exhibited structural differences (i.e. differences in frequency ratios within and between
notes) from those of the black-capped song, though the form
of these song variants were not consistent across the range
(Grava et al. 2013a).
Whilst research by Grava et al. (2012a, 2013a) suggests that mountain chickadees alter song variants in areas of
sympatry with black-capped chickadees, no studies have examined whether these sympatric song variants result in fewer
negative interactions with dominant black-capped chickadees,
as seen in European and Asian tits (Doutrelant et al. 2000;
Hamao et al. 2015). Following the study design of Doutrelant et
al. (2000), which examined the behavioural response of great
tits to Eurasian blue tit vocalisations recorded in areas of sympatry and allopatry, we tested the hypothesis that mountain
chickadees alter their vocalisations to minimise the negative
interactions from black-capped chickadees. We predicted that
black-capped chickadees unfamiliar with mountain chickadee
song would respond less aggressively to the mountain chicka-

1. METHODS AND MATERIALS
Playback Construction. We created a series of 10 playback
dyads (paired playbacks, one playback with sympatric songs
and one with allopatric songs) using mountain chickadee
songs from populations across British Columbia and the western United States. Stimuli were obtained from the study by
Grava et al. (2013a). ‘Allopatric’ playbacks consisted of mountain chickadee songs taken from a region where black-capped
chickadees were not present whilst ‘sympatric’ playbacks consisted of mountain chickadee songs recorded from areas where
both chickadee species coexist. For each pair of stimuli, songs
were taken from allopatric/sympatric populations within a similar region (e.g. sympatric populations from northern California
vs. allopatric populations in central and southern California, or
known sympatric or allopatric populations within British Columbia). Stimuli were chosen from recordings that had high signal-to-noise ratios; from these, three or four high-quality songs
were extracted and used to make each playback stimuli in the
dyadic pairs. Each playback consisted of 12 songs per minute,
with the three or four exemplar songs randomly interspersed
in the sequence. Songs were spaced approximately 5–6 s apart,
for a total of 24 songs over a total of 2 min. Each song used in
creating the playbacks was standardised for amplitude using
Avisoft SASLab Pro software (Specht 2012). We used Audacity
software (Audacity Team 2008) to sequence the three to four
exemplar songs into each playback stimulus, either allopatric
or sympatric, to create variation whilst still representing successive songs of a single stimulus male. Low levels of Brownian
noise (1% amplitude) were added to the playbacks with Audacity to diminish noises introduced during the editing process.
The completed stimulus included a 30-s blank intro followed
by 2 min of stimulus broadcast. Unique dyads were created by
randomly pairing one ‘allopatric’ stimulus with a ‘sympatric’
stimulus, matched per region of stimulus origins. Four dyadic
pairs were made from recordings from the western United
States, and six stimulus pairs were made from recordings made
throughout British Columbia, Canada. All dyads were loaded
onto an Apple iPod Touch for field playback and broadcast
through a Logitech X100 Bluetooth speaker. Each dyadic playback set was calibrated to a sound level of 75 dB at 1 m during
playback, measured with a Goldline SPL-162 Sound Pressure
Level Meter.
Field Methods. Playback experiments were conducted in Prince George, BC, Canada (53°54’40.1”N 122°45’18.6”W),
from 21–29 April, 2015, following the methods of Doutrelant et
al. (2000). We conducted playback experiments on a total of
22 unbanded black-capped chickadees in an allopatric population, at 22 separated locations around the University of Northern British Columbia campus, the outskirts of Prince George
and within the Prince George city park, Forests for the World.

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EUROPEAN JOURNAL OF ECOLOGY

Males were equally distributed amongst four main sites that
were separated by a minimum of 3 km and a maximum of 7
km. Within sites, males tested on the same day were separated
by at least 250 m to avoid testing neighbours who might have
been eavesdropping on the playbacks (McGregor 1992, 2002;
McGregor et al. 1992). Natural song perches were used to determine locations of territory centroids, from which we conducted playbacks.
Playbacks were performed in the morning (07:00
to 12:00 am). Two to four 10-m long ropes, with 5 m markers, were set up equal distances apart radiating out from the
middle of each focal bird’s location, with a Logitech X100 Bluetooth speaker hanging in the centre, approximately 1 m off the
ground. The speaker faced the direction of the focal bird and
was connected via Bluetooth or using a plug-in adapter directly
to the iPod.
Each focal chickadee was given a randomly selected
playback stimulus (either allopatric or sympatric), followed
by a second playback stimulus of the alternative type 1–2 h
later at the same location, resulting in a total of two behavioural recordings per bird. The next focal bird was given a set
of playback stimuli in the opposite order to randomise trials
and avoid playback bias resulting from order effects (McGregor
1992, 2002; McGregor et al. 1992). In each playback trial, focal birds were first called in with a ‘primer’ call (a short playback of mobbing black-capped chick-a-dee calls) and were then
given the selected playback recording. Behavioural responses
were observed and recorded vocally using an Audiotechnica
AT8015 microphone and a Marantz PMD661 MKII Professional
Portable Flash Field Recorder. We verbally dictated the focal
male’s distance from the speaker at all points during the trials, noting each instant the male moved to a new location. We
noted all movement flights as either short (<5 m) or long (>5
m). Further, we indicated whether or not the mate was present and her position relative to the subject male. Finally, we
recorded all vocalisations made (gargles, chick-a-dee calls or
songs) throughout the trials. Each playback dyad was played to
either two (N = 8) or three (N = 2) males only, in reversed order,
as recommended by McGregor et al. (1992) to avoid issues of
pseudoreplication.
Audio Analysis. Recordings were individually annotated using Avisoft SASLab Pro bioacoustics software, taking
note of focal bird’s location and behaviours mentioned earlier
throughout the playback trial. These annotations were then
exported into text files. The observations noted in the text files
were processed and summarised using R v3.3 statistical software (R Core Team 2016). Specifically, we extracted information on the focal male’s distance from the speaker playback,
time spent at each distance, how many vocalisations were
made and the level of aggressiveness with relation to the
stimuli they were exposed to (whether the focal male song was
non-overlapping, overlapping or overlapped in relation to the
playback stimuli) and time spent within 10 m of the speaker
after the playback ended. Only birds with complete recording

files of the trial were used in the analysis; incomplete or interrupted recordings were excluded from analysis (nincomplete = 2).
Statistical Analysis. We used R v.3.3 statistical software to conduct iterative principal component analysis (PCA)
to collapse the variables into two holistic measures, one describing approach behaviour and the other describing vocal
response. Next, we constructed generalised mixed models in
JMP 12 (SAS Institute 2015) to determine whether approach
response or vocal responses varied with respect to the playback presented (allopatric/sympatric). We included playback
order and starting distance (as well as an interaction term) as
covariates. Male ID was included as a random effect to account
for the paired design.

2. RESULTS
Principal Component Analysis. For each trial, we tested combinations of behaviours associated with both individual approach and vocal responses to playback within PCA. Variables
that were intercorrelated, or did not explain much variance,
were removed so that we could make a parsimonious model
that explained variation in male response with the most concise number of contributing variables. We collapsed time spent
within several distinct radii of the playback speaker (e.g. distance from 0 to 5 m and distance from 5 to 10 m were collapsed
to distance from 0 to 10 m) to derive a total of six variables, for
which the first two principal components (PCs) explained 70%
of the variance (Table 1). Both PC1 and PC2 met the broken
stick criteria of components that explained a greater proportion of the variation than that explained by chance alone (Legendre and Legendre 1998). PC1 (hereafter called ‘approach response’) was most strongly associated with variables related to
focal male distance from speaker during the playback period,
with negative weightings for minimum distance (i.e. higher PC1
values indicate that birds were approached more closely; Table 1), and time spent at >20 m from the speaker, but positively
weighted for time spent close to the speaker (time spent at
0–10 m). Also, there was a weak negative weighting on singing
fewer overlapping songs. Thus, higher values of PC1 indicate
close and sustained approaches to the speaker. PC2 was negatively associated with variables related to focal male vocalisation responses to the stimuli during the playback period (i.e.
singing and overlapping songs more; Table 1) and positively
with time greater than 20 m from the speaker; so high values
of PC2 represent birds being distant and singing and overlapping little in response to playback. We subsequently multiplied
the PC2 values by −1 to reverse the axis, so that greater values
of PC2 indicate a more ‘aggressive’ singing response.
PC1 – Approach Response. We analysed paired responses from 20 individuals (40 observations total). Blackcapped chickadees did not differ in approach response when
presented with allopatric versus sympatric mountain chickadee
song (F1,19 = 0.34, p = 0.57; Fig. 1). Because the approach response may have been influenced by the initial starting location
of the black-capped chickadee, we included starting distance

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Table 1. Principal component analysis results of measures of distance from the speaker and song response variables. PC1 was most strongly associated with the approach
variables, whilst PC2 was most associated with vocal responses. Bolded values reflect variables with contributions of greater than 0.33 or less than −0.33, which are
considered to make a substantial contribution to the axis (Ho 2006). Note that for PC2, the values were subsequently multiplied by −1 to create an index of increasing
song response.

Variable

PC1 Factor Loading

PC2 Factor Loading

Minimum distance to speaker

−0.55

0.21

Distance 0–10 m

0.55

−0.08

Distance 10–20 m

−0.24

−0.24

Distance >20 m

−0.45

0.36

Total number of song responses

−0.12

−0.69

Total overlapping song responses

−0.34

−0.54

Total variance explained

0.44

0.26

.

Figure 1. Response of black-capped to chickadees to allopatric and sympatric mountain chickadee songs. Neither black-capped chickadee PC1 approach (A) nor PC2 song (B) differed between allopatric and sympatric playback trials.

responses). Neither the interaction term nor the order effect
were significant (first trial: F1,18 = 0.19, p = 0.67; playback type:
t1,18 = 0.32, p = 0.58; first trial*playback type: F1,18 = 0.24, p =
0.63). Upon removal of the interaction term, there was still no
effect of either playback order or playback type (first trial: F1,18
= 0.19, p = 0.67; playback type: F1,19 = 0.34, p = 0.57). When we
included starting distance in the model, there was a significant
effect of starting distance; however, playback trial and order
were not significant (first trial: F1,16.6 = 0.26, p = 0.62; playback
type: F1,18.3 = 0.03, p = 0.87; starting distance: F1,26.9 = 55.52, p =
< 0.0001).
PC2 – Song Response. We found no difference in song
response to the type of playback presented to black-capped
chickadees (allopatric vs. sympatric; t1,19 = 0.68, p = 0.42; Fig. 1).
As with approach response, we included starting distance as
a covariate and an interaction between starting distance and

as a covariate and an interaction between starting distance
and playback in the model. As anticipated, starting distance
had a strong effect, but there remained no effect of playback
type, nor was there an interaction between starting distance
and playback type (starting distance: F1,28.1 = 51.73, p < 0.0001;
playback type: F1,17.3 = 0.03, p = 0.85; starting distance*playback
type: F1,28.3 = 1.57, p = 0.22). Because the interaction term was
not significant, it was removed from the model. Starting distance remained significant, but there was no significant effect
of playback type (starting distance: F1,27.9 = 59.64, p < 0.0001;
playback type: F1,18.3 = 0.03, p = 0.87).
Next, we generated a model with an additional effect of playback order (i.e. whether allopatric was presented
first or second) and an interaction term to determine whether
there was an order effect (i.e. whether the song played first [allopatric or sympatric] influenced the black-capped chickadee

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playback type in the model. There was no effect of starting distance, playback type, or the interaction term (starting distance:
F1,24.8 = 1.97, p = 0.17; playback type: F1,18.4 = 0.85, p = 0.37;
playback type*starting distance: F1,33.3 = 1.16, p = 0.29). Consequently, the interaction term was removed from the model.
There remained no effect of either starting distance or playback type (starting distance: F1,24.7 = 1.46, p = 0.24; playback
type: F1,19.0 = 0.79, p = 0.39).
Next, we produced models that included playback
order and an interaction term. Playback order, playback type,
and the interaction term were all not significant (first trial:
F1,18 = 0.49, p = 0.49; playback type: F1,18 = 0.65, p = 0.43; first
trial*starting distance: F1,18 = 0.12, p = 0.74) and remained
non-significant after the removal of the interaction term (first
trial: F1,18 = -0.49, p = 0.49; playback type: F1,19 = 0.68, p = 0.42).
An additional model was created to examine playback order
as well as starting distance; however, there were still no significant effects (first trial: F1,17.3 = 0.42, p = 0.53; playback type:
F1,19.1 = 0.78, p = 0.39; starting distance: F1,23.6 = 1.34, p = 0.26).

3. DISCUSSION
In contrast to studies on European and Asian tits, we did not
observe that the song variants of mountain chickadees used in
regions of sympatry reduced aggressive responses from naïve
black-capped chickadees more than the songs from allopatric
populations. Black-capped chickadees in our study area did
not appear to differentiate between sympatric and allopatric mountain chickadee songs in any of their behavioural responses. During both sympatric and allopatric playback trials,
black-capped chickadees reacted by approaching the speaker
and vocalising during the trial. Although we did not include a
conspecific stimulus in this study – for fear of inducing habituation to too many stimuli – the responses of subject males in this
study were qualitatively similar in approach distance and time
spent close to the speakers as seen in previous studies when
black-capped chickadee males were presented conspecific song
in non-interactive playbacks (Shackleton et al. 1992; Otter et al.
2002; Mennill and Ratcliffe 2004; Grava et al. 2013b). Thus, our
results suggest that black-capped chickadees recognise both
types of mountain chickadee song stimuli and respond to them
as a potential threat. The song variants, or character-shifted
songs, exhibited by mountain chickadees in populations where
the two species are sympatric (Grava et al. 2013a) did not appear to minimise the responses of naïve black-capped chickadee males. This result is unexpected because of the findings of
studies on pairs of other Parid species in which the subordinate
species do display character shifts that reduce negative interactions with the more dominant form (Doutrelant et al. 2000).
Black-capped chickadees respond less aggressively to
heterospecific calls than to conspecific chick-a-dee calls (Grava et al. 2012b). However, when we presented black-capped
chickadees with only heterospecific mountain chickadee songs,
there was an equal reaction to both the allopatric and sympatric versions of this song. One possibility is that the sympatric

populations of mountain chickadees from which we obtained
stimuli are in the early stages of character displacement; the
structure of their songs may still be very similar to that of
mountain chickadees in allopatric populations. It may continue
to change until it becomes less recognisable to black-capped
chickadees, and at that point, it may be perceived as less of a
threat.
Owing to the natural segregation of both species
based on habitat type, there tend to be small areas of overlap
rather than large zones. The existence of these isolated areas
may result in independent character shifts within each of the
distinct sympatric populations (Grava et al. 2013a), that is, a variety of different character-shifted song variants occur amongst
disparate sympatric populations. Because chickadees rely on
learning for song development, cultural evolution and the
rapid emergence of local dialects within sympatric populations
of mountain chickadees may limit the ability of black-capped
chickadees to recognise and respond to non-local mountain
chickadee song variants. LaZerte et al. (2016) demonstrated
that male black-capped chickadees required prior exposure to
urban noise in order to respond appropriately (shift song upwards in pitch) to playback of experimental noise; it is possible
that parallel prior experience with mountain chickadees, and
learning the local character-shifted song variant, is required
to see reduced responses in male black-capped chickadees.
As we tested only black-capped chickadees from an allopatric
population, naïve to mountain chickadee song, this may account for the lack of discrimination. Future playbacks on male
black-capped chickadees in overlapping populations, testing
local sympatric mountain chickadee song variants versus those
arising from other sympatric populations, might elucidate this
idea.
Alternately, the character-shifted songs seen in sympatric mountain chickadee populations may not have evolved
to reduce aggression from black-capped chickadees but rather
may arise owing to female mate choice. In black-capped chickadees, females prefer dominant males as social and extra-pair
partners (Ramsay et al. 2000) and can learn the relative rank of
the male based on non-pitch-based cues in vocalisations and
respond more to dominant male songs (Hoeschele et al. 2010).
Ratcliffe and Otter (1996) demonstrated that females exhibit
reduced response to songs with incorrect internote intervals
and flattened within-note frequency ratios in the fee note. Additionally, Christie et al. (2003) showed that songs are highly
stabilised and that the ability to maintain consistency is a dominance-related characteristic. Because social hierarchies govern social behaviour, interspecific hierarchies could potentially
drive hybridisation if females are choosing dominant males as
extra-pair partners regardless of species (Grava et al. 2012a).
Both black-capped and mountain chickadees are monogamous
and form season-long pair-bonds; however, Grava et al. (2012a)
found that in regions of overlap, black-capped males sire the
majority of mountain chickadee nestlings through extra-pair
copulations, rather than through social pairings (Grava et al.
2012a). This pattern can also be seen in hybrid zones between

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Carolina chickadees Poecile carolinensis and black-capped
chickadees, where female black-capped chickadees prefer
dominant male Carolina chickadees (Bronson et al. 2003; Reudink et al. 2006). As the subordinate species, male mountain
chickadees may have reduced expression of a favoured dominance trait, leading to mating of the female mountain chickadees with the dominant male black-capped chickadees. This
could reduce the potential reproductive output of male mountain chickadee (Grava et al. 2012a, 2013a). If variant mountain
chickadee songs are perceived as more attractive by females,
selection could favour altered vocalisations in regions of sympatry if males expressing this song are better able to attract
mates and avoid paternity loss from black-capped chickadees.
Our results do not follow the pattern of character
shifts acting to reduce aggression from dominant species, as
observed in Asian and European tits, close relatives of North
American chickadees. Repeating this study on other blackcapped chickadee populations, particularly those already sympatric with mountain chickadees, may prove informative. The
form of character displacement in song divergence of mountain chickadees varies across sympatric populations, which
may suggest that black-capped chickadees require a period
of song learning to recognise local song variants. As such, we
would predict that only black-capped chickadees in sympatric
populations may exhibit a differentiated response and that
black-capped chickadees may only reduce aggression towards
the local mountain chickadee variant. In addition, it may be
beneficial to control for black-capped chickadee response to
black-capped songs (i.e. conspecific song). Presenting a male
black-capped chickadee with both a black-capped song (control) and either an allopatric or sympatric mountain chickadee
songs would allow black-capped chickadees to compare be-

tween conspecific and heterospecific songs, potentially resulting in greater differentiation between sympatric and allopatric
songs. Furthermore, incorporation of Carolina chickadee vocalisations, to which black-capped chickadees in western Canada
have not been exposed, into a playback presented to blackcapped chickadees may allow us to interpret whether blackcapped chickadees are merely responding to vocalisations that
exhibit ‘conspecific-like’ attributes or if they do in fact differentiate between species. Finally, mate choice trials in which female mountain chickadees are presented songs from sympatric
and allopatric populations could indicate whether song shifts
exhibited by mountain chickadees are driven by sexual selection pressures.
Our results demonstrate that mountain chickadee
songs with sympatric song variants do not reduce heterospecific aggression from black-capped chickadees. This finding is
particularly interesting as, to our knowledge, within the family
Paridae, it is the only instance of a character shift being unassociated with reduced aggression. Our results raise interesting
questions about the selective pressures leading to the evolution of this song divergence. It is possible that different selective pressures (e.g. social, sexual) may result in similar evolutionary outcomes in the form of altered songs in sympatric
populations.
Acknowledgements: The authors thank S. Joly, N. Flood, K.
Marini, T. Dickinson and J. Kusack for providing feedback and
discussion that improved this manuscript. Funding was provided by a CUEF U-REAP Undergraduate Research Grant to C.L.S.
and Natural Sciences and Engineering Research Council Discovery Grants to M.WR. and K.A.O.

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