CHARACTER RELEASE IN BIRD SONG - A TEST OF THE ACOUSTIC COMPETITION HYPOTHESIS USING AMERICAN TREE SPARROWS SPIZELLA-ARBOREA

The acoustic competition hypothesis predicts that song variability wit hin 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 w ithin a 500 km2 area. Most of these locations contained a different as semblage of sympatric passerines, thus representing different sound en vironments. We estimated the complexity of the sound environment by th e number (richness) of passerines we found singing at each location. E ach male American Tree Sparrow sang a single, stereotyped song. We qua ntified song variation as song type richness at each location. In supp ort of the acoustic competition hypothesis, we found a strong inverse relationship between song variability (song type richness) and sympatr ic species richness. This is the first study to show a relationship be tween song type variation and the complexity of the acoustic environme nt. We suggest that this pattern could result from males of this speci es choosing territories in areas where their song type is most effecti ve.