Habitat-related differences in the frequency selectivity of auditory filters in songbirds

1. Environmental constraints in woodland habitats favour long-range communication signals with slow modulations of frequency and amplitude, while constraints in open habitats favour faster modulations. 2. Spectral filtering by the peripheral auditory system results in a trade-off between frequency resolution and temporal resolution of modulations. Greater frequency resolution requires integration of acoustic signals over a longer period of time, which in turn decreases responsiveness to temporal modulations. 3. Here, we test the hypothesis in songbirds that woodland habitats have selected for narrowly tuned auditory filters with high frequency resolution of tonal elements, while open habitats have selected for broader auditory filters with greater temporal resolution of rapid modulation. 4. Auditory filter shapes were measured at 2, 3 and 4 kHz in three woodland species, the dark-eyed junco (Junco hyemalis), tufted titmouse (Baeolophus bicolor) and white-breasted nuthatch (Sitta carolinensis), and two open-habitat species, the house sparrow (Passer domesticus) and white-crowned sparrow (Zonotrichia leucophrys). Auditory filter shapes were derived from auditory brainstem response thresholds in notched noise using Patterson's rounded exponential (roex) auditory filter model. 5. Auditory filters increased in bandwidth with increasing frequency in all species except the dark-eyed junco. Auditory filters were generally narrower in woodland species than open-habitat species as predicted, although auditory filters of the junco were relatively narrow only at 4 kHz, and the difference between white-crowned sparrows and tufted titmice was not significant. Finally, at 4 kHz, open-habitat species had auditory filters with lower signal-to-noise response thresholds than woodland species (i.e. greater response efficiency). 6. The results suggest that environmental constraints on song structure have influenced the evolution of peripheral auditory filters in songbirds, although species using signals not optimized for long-range transmission should be less affected. Differences in filter bandwidth between dark-eyed juncos and white-crowned sparrows are noteworthy because Junco and Zonotrichia share a recent common ancestor as sister genera within the Emberizidae. Finally, open-habitat species may compensate for inherently lower sensitivity in noise of broad auditory filters with greater response efficiency.