DOI: 10.5061/dryad.3bk3j9kts
title: Song measure averages of allopatric and sympatric Black-capped and Mountain Chickadee individuals

description: Character displacement theory predicts that closely related co-occurring
 species should diverge in relevant traits to reduce costly interspecific
 interactions such as competition or hybridization. While many studies
 document character shifts in sympatry, few provide corresponding evidence
 that these shifts are driven by the costs of co-occurrence. Black-capped
 (Poecile atricapillus) and Mountain Chickadees (P. gambeli) are closely
 related, ecologically similar, and broadly distributed songbirds with both
 allopatric and sympatric populations. In sympatry, both species appear to
 suffer the costs of their co-occurrence: 1) both species are in worse body
 condition compared to allopatry and 2) hybridization sometimes yields
 sterile offspring. Here, we explored character displacement in the songs
 of Black-capped and Mountain Chickadees by characterizing variation in
 male songs from sympatric and allopatric populations. We found that
 Mountain Chickadees sing differently in sympatry versus allopatry.
 Specifically, they produced more notes per song, were more likely to
 include an extra introductory note, and produced a smaller glissando in
 their first notes compared to all other populations. Combined with
 previous research on social dominance and maladaptive hybridization
 between Black-capped and Mountain Chickadees, we posit that differences in
 sympatric Mountain Chickadee songs are population-wide shifts to reduce
 aggression from dominant Black-capped Chickadees and/or prevent
 maladaptive hybridization.
description: <em>Field sampling</em> During
 April and May of 2020, we recorded breeding Black-capped and Mountain
 Chickadees in both sympatry and allopatry. We recorded sympatric
 populations in Boulder County, Colorado, an area where hybridization is
 known to occur and we sampled along both species’ elevational
 distributions (Black-capped Chickadee n = 21; Mountain Chickadee n = 11).
 We recorded allopatric Black-capped Chickadees in Ithaca, New York (n =
 21), and allopatric Mountain Chickadees near Truckee, California (n = 20).
 In both allopatric populations, neither species overlaps with any other
 Chickadee species. To avoid repeat sampling, we walked at least 500 meters
 between recordings and never sampled in the same location twice. We
 collected a total of 2,080 songs and an average of 29 songs per
 individual. All individuals produced a minimum of six songs. We recorded
 songs with a PMD660 or PMD661 Marantz digital recorder and a Sennheiser
 ME-66 unidirectional microphone with a sampling rate of 44,100 Hz and a
 16-bit resolution.  <em>Measuring song
 characteristics</em> We analyzed songs using the
 bioacoustics software Raven Pro 1.6.3. We formatted spectrograms in the
 color scheme “Jet” with the x-axis scaled at 0.1 second intervals and the
 y-axis scaled at 0.5 kHz intervals. To control for variation in background
 noise levels and recording distance from birds, we adjusted contrast and
 brightness on each recording to establish consistency among all
 recordings. To control for researcher subjectivity, only one researcher
 took all song measures. As such, brightness and contrast were adjusted
 somewhat subjectively, but consistently by the observer to optimize
 visualization of the song. For all songs we measured song duration,
 frequency ratio, and internote interval, which was calculated post hoc
 (Table 1). For each note we measured duration, glissando, frequency 5%,
 and frequency 95%. To avoid pseudoreplication, we calculated the mean of
 each song measure per individual across all of their songs, such that each
 individual is represented only once in downstream statistical analyses.
 For each song from an individual, we annotated song number, note number,
 and whether or not the song included introductory notes. These song
 measures are based on previous research on Black-capped and Mountain
 Chickadee song. No recordings contained songs with more than three
 introductory notes. We omitted faint noise or “echoes” at the beginnings
 and ends of notes, which resemble smears of a consistent,
 recording-specific, length in the spectrograms. As such, notes that
 appeared to be connected by an amplitude dip rather than separated by a
 true break were selected as two separate notes; we treated the amplitude
 dips as omittable echoes and selected them as different notes. While our
 analysis methods differ from traditional chickadee literature (48), which
 treat notes connected by an amplitude dip as a single note, we treated all
 populations the same so results are not affected. Importantly, this aspect
 of song does not drive any of the differences we report. 
 <em>Statistical analysis</em> We
 performed all statistical analyses in R version 4.0.3 using the packages
 tidyverse and ggplot2. To assess whether chickadee songs exhibit more
 divergence in sympatry, as predicted from character displacement theory,
 we used a principal component analysis (PCA) to assess if songs cluster
 together by population. We used individual means for all song measures in
 the principal component analysis. Then, to assess what features of song
 explained the most variation between and within populations, we ran linear
 models (command lm()) for each PC axis that had a loading stronger than
 0.3, with population (Allopatric BCCH, Sympatric BCCH, Allopatric MOCH,
 and Sympatric MOCH) as the dependent variable. To assess variation between
 populations, we ran ANOVAs (command aov()) on each linear model followed
 by a Tukey HSD test to quantify differences between populations.
 Statistical significance was set at p &lt; 0.05. 
description: # **Song measure averages of allopatric and sympatric Black-capped and
 Mountain Chickadee individuals**
 [https://doi.org/10.5061/dryad.3bk3j9kts](https://doi.org/10.5061/dryad.3bk3j9kts) This data contains the averages of 14 song measures per 72 mountain and black-capped chickadee individuals, in addition to the number of songs each individual sang and whether they came from a sympatric or allopatric population. This data also contains the 72 song files we used to compile the averages. We ran a principal component analysis (PCA) to assess if songs cluster by population using individual averages for all song measures. To assess which song measures explained the most variation between and within populations, we ran linear models for each PC axis with a loading stronger than 0.3, with the population as the dependent variable. Then, to assess variation between populations, we ran analyses of variance (ANOVAs) on each linear model followed by a Tukey HSD test to quantify differences between populations. We found that sympatric Mountain Chickadees sing a higher note number per song, are more likely to include two introductory notes, and produce a smaller glissando across the first note (excluding introductory notes) compared to all other populations.  ## **Description of the data and file structure** The zip folders contain the song files. BCCH_allo contains allopatric black-capped chickadee songs, BCCH_sym contains sympatric black-capped chickadee songs, MOCH_allo contains allopatric mountain chickadee songs, and MOCH_sym contains sympatric mountain chickadee songs. In the averages csv, the column ‘Unique_ID’ contains the species (black-capped chickadee or mountain chickadee), population (sympatric or allopatric), and numeric ID for each individual, and allows for each individual to be represented and plotted in the data. ‘Species_Geo’ contains the species and population for each individual and allows the clustering of groups by species and/or population.  Here is a table that describes the remaining columns in the averages csv: | **Column** | **Description** | | :---------------------- | :------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | numb\_notes | The average number of notes per song | | intro\_*n* | The proportion of songs per individual with *n* introductory notes present; *n* = 1-3 | | song\_duration (s) | The total length of each song  | | note\_dur (s) | The length of each note, excluding introductory notes | | freq\_shift (Hz) | The major frequency shift per song; calculated as the first note of the song divided by the second note, excluding introductory notes | | n1\_gliss (Hz) | The difference between the highest and lowest frequencies per note; taken for the first note of the song excluding introductory notes | | n2\_gliss (Hz) | The difference between the highest and lowest frequencies per note; taken for the second note of the song excluding introductory notes | | internote\_interval (s) | The duration between each note | | *n*\_freq\_5 (Hz) | From Raven Pro: the frequency that divides the selection into two frequency intervals containing 5% and 95% of the energy in the selection; taken for the first two notes of the song excluding introductory notes. | | *n*\_freq\_95 (Hz) | From Raven Pro: the frequency that divides the selection into two frequency intervals containing 95% and 5% of the energy in the selection; taken for the first two notes of the song excluding introductory notes.  | ## **Change log** 21 September 2024: In previous versions, I did not upload the song files. Now they are included in the relevant zip folders.