Testing the acoustic adaptation hypothesis for eastern and spotted towhee song

Abstract

The acoustic adaptation hypothesis predicts that signals adapted for long-distance communication in forested habitats should possess low rates of amplitude modulation (slow trills), whereas signals adapted to open habitats should possess high rates of amplitude modulation (fast trills). The songs of two songbirds, the eastern and spotted towhees, were used to test this hypothesis. These two species have only recently been recognized as distinct species. Differences in song are one of the reasons given for splitting the species. Eastern towhees occupy edges of deciduous forests and sing slow trills. Spotted towhees occupying open chaparral habitat sing fast trills. Songs of both species were broadcast in their native habitats, and recorded at several distances from the sound source. Spectrogram cross-correlation was used to quantify sound degradation with distance. The results support the predictions of the acoustic adaptation hypothesis. The slow eastern towhee trill degrades less than the fast spotted towhee trill in a forested habitat, whereas the fast spotted towhee trill degrades less than the slow eastern towhee trill in an open habitat.