The relationship between frequency overlap and temporal acoustic avoidance in a temperate North American bird community

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Lauren M. Chronister, Tessa A. Rhinehart and Justin Kitzes, Department of Biological Sciences, University of Pittsburgh, Pittsburgh, PA

Background/Question/Methods
In crowded communities, birds face the challenge of making themselves heard. Temporal overlaps between a bird’s own acoustic signals and those of others may obscure its messages, so many birds have adopted a strategy of dynamically timing their songs to avoid temporal overlap with other birds. However, birds could also exploit frequency range differences between vocalizations to sing synchronously while minimizing interference. It has been hypothesized that birds should most strongly avoid temporal overlap when they overlap highly in frequency. Using 365 minutes of continuous audio recording from three passive acoustic monitors, we investigated a community of temperate North American birds for evidence of a positive correlation between frequency overlap and temporal avoidance. We created a null model of temporal overlap between species pairs by randomizing the timing of observed vocalizations and compared this with the true observed overlap to generate a temporal avoidance score. We also calculated the average proportional frequency overlap between pairs of species. We then conducted a series of permutation tests using Pearson’s correlation coefficients as the test statistic to evaluate the statistical significance and direction of the relationship between frequency overlap and temporal avoidance scores for each species and the community as a whole.
Results/Conclusions
We found significant evidence supporting the hypothesis that birds will increase their temporal avoidance of one another as their frequency overlap increases in a temperate bird community. When all possible pairs of species sampled from a recorder were considered at once, there was a significant positive correlation between frequency overlap and temporal avoidance in all three recordings (p < 0.0001). Furthermore, after performing the Benjamini-Hochberg procedure for multiple comparisons (q* = 0.05), several tests evaluating the correlation between frequency overlap and temporal avoidance within individual species were also significant. For each of these tests, we considered only a single species and all of its possible heterospecific pairs. In all, 9 of 48 species sampled showed a significant positive correlation between frequency overlap and temporal avoidance, and 89% of correlation coefficients were positive. No significant negative correlations were found. These findings support previous work in some tropical bird communities where birds with high-frequency songs seem to avoid the choruses of high-frequency cicadas and overlap each other less than expected in more crowded frequency bands. Future extensions of this work could experimentally evaluate temporal overlap avoidance by a focal species using playback that varies in frequency overlap with the focal species’ songs.