Character Release in Bird Song: A Test of the Acoustic Competition Hypothesis Using American Tree Sparrows Spizella arborea

Abstract

The acoustic competition hypothesis predicts that song variability within a species will be inversely correlated with the amount of acoustic competition from other species. To test this hypothesis, we recorded 10 territorial male American Tree Sparrows from each of 14 locations within a 500 km2 area. Most of these locations contained a different assemblage of sympatric passerines, thus representing different sound environments. We estimated the complexity of the sound environment by the number (richness) of passerines we found singing at each location. Each male American Tree Sparrow sang a single, stereotyped song. We quantified song variation as song type richness at each location. In support of the acoustic competition hypothesis, we found a strong inverse relationship between song variability (song type richness) and sympatric species richness. This is the first study to show a relationship between song type variation and the complexity of the acoustic environment. We suggest that this pattern could result from males of this species choosing territories in areas where their song type is most effective.