Supplementary materials
Call recordings and measurements. For the Himalayan species, we obtained two
recordings from different individuals for 17 of the species and one for the other 5. We
recorded calls produced in response to taxidermied predator mounts of either locally
occurring sparrowhawk (Accipiter nisus) or a congeneric of a locally occurring pygmy
owl (mount: Glaucidium passerinum; locally occurring species: G. brodiei). Additionally,
we obtained high-quality recordings of the calls used during mobbing by 9 species of
New World songbirds to evaluate responses to distantly allopatric species (figure S2;
table S3). One unique call recording for 7 species and two from an additional two species
were obtained from Xeno-Canto, an online database of bird vocalizations (www.xenocanto.org). We edited all recordings using RAVEN (v. 1.3) to eliminate background
noise, to standardize the amplitude to natural levels when played though a field speaker
(75 dB at 5 m; recorded by a Radio Shack Sound Level Meter, model 33-2055), and to
standardize the length of the call recordings to 10 minutes.
We took 10 acoustic measurements in RAVEN (v. 1.3) of the first two calls of
each individual that were of high quality and free of background noise from other animal
vocalizations and wind (figure S1). We used a Hann window function, a temporal grid
resolution of 2.90 ms with 50% overlap, and a frequency grid resolution of 172 Hz. To
obtain measurements, we standardized the frequency range (0 – 10 kHz) and time scale
(1 second) of the spectrogram view window in RAVEN. We determined high- and lowfrequency as well as start- and end-time cutoffs as the point at which the amplitude of the
spectrogram falls below 10 dB of the maximum amplitude of the call. Five species
(Parus ater, Parus rufonachalis, Phoenicurus coeruleocephala, Tarsiger chrysaeus, and
Tarsiger rufilatus) have more than one distinct call type (figure S4). For analyses of
similarity across species, we used the call type most similar (i.e., least mean Euclidean
distance) to those of the other species.
Call variation in Phylloscopus t. trochiloides. Whereas the call repertoires of other
species included in the study are composed of single, stereotyped call types or between
two and five acoustically distinct types (figure S4), the calls of P. trochiloides (sbsp.
trochiloides) have a number of subtle variants that are acoustically similar (figure 2). We
visually assessed five minutes of continuous call recording (more than 300 calls per
individual) from 10 P. t. trochiloides individuals from Manali and 7 from Keylong to
obtain an estimate of the number of call variants per individual. The population of P. t.
trochiloides at Keylong coexists and interbreeds with individuals from a distinct
mitochondrial clade, classified as a different subspecies, P. trochiloides viridanus (Irwin
et al. 2008), and P. t. trochiloides individuals at Keylong sometimes produce P. t.
viridanus-typical calls. We excluded P. t. viridanus-typical calls for our analysis, but the
results were similar when they were included. We visually compared the resultant call
variants across individuals and across sites (figure 2) in order to evaluate the degree of
call sharing.
P. t. trochiloides individuals produce calls incorporating the call of Phylloscopus
pulcher in “runs” intermixed with calls that do not incorporate the call of P. pulcher. To
create recordings of these two call categories for the playback experiments, these runs
were either isolated from or edited out of longer recordings. We broadcast each playback
twice at Manali, where both species co-occur. Responses to each pseudo-replicate were
similar and, thus, we analyzed only the results from the first replicate to allow
comparison with other playback experiments (table 2).
Playback experiment protocol. We transferred recordings to an iPod (Apple; Cupertino,
CA) and played them for 10 minutes through a Mineroff SME-AFS Portable Field
Speaker (Mineroff Electronics Inc.; Elmont, NY). A given playback was conducted at
least 100 m from the previous experiment to avoid pseudoreplication. For each playback,
we placed the speaker in a camouflaged location within the branches of a tree at a height
of ~2 m and hid behind a field blind. We started the playback and recorded the response
of surrounding birds by monitoring nearby bushes and trees within 20 m of the speaker.
We noted the number of respondents, their species identities, and their behavior. We
evaluated the length of time an individual responded, the minimum distance it
approached the speaker, and whether it engaged in typical mobbing behaviors such as
flicking its wings and calling.
Figure S1. Spectrogram display showing how 10 call measurements were obtained for
two representative calls of (a) Phylloscopus t. trochiloides and (b) Seicercus whistleri.
Duration, bandwidth, and high and low frequencies of the call were manually selected
and the other measurements were exported from Raven (v. 1.3). Blue dashed lines denote
3rd quartile, center, and 1st quartile frequencies, the yellow dot denotes peak frequency,
and vertical red lines denote frequency modulations. The call of Seicercus whistleri is
composed of two notes.
Figure S2. Spectrogram display of the calls of 9 American species used in the study. The
“chick-a-dee” call of Poecile atricapillus is ~ 1.5 s long, so is presented on a different
scale.
Figure S3. Spectrogram display of the mobbing calls of (a) Phylloscopus occipitalis, (b)
Phylloscopus t. trochiloides, and (c) Phylloscopus humei that we manipulated in Raven
(v. 1.3) to incorporate the call of P. occipitalis.
Figure S4. Spectrogram display of mobbing call repertoires of 5 species found to use
more than one call type. *Call types that were found to have the smallest average
Euclidean distance to call types of the other species and, therefore, were included in
subsequent analyses.