Discrimination of song dialects in relation to song similarity and geographical distance in a rainforest passerine

Abstract

Whether or not songbirds recognise geographical variation in song is thought to depend on the perception of threat of neighbours versus strangers or the geographical scale at which song changes in relation to an individual's dispersal distance, or both of these combined. We aimed to test the perception of different song-variants in the Chowchilla (Orthonyx spaldingii), a rainforest-restricted Australian songbird that shows strong geographical variation in song, thus providing an ideal system to test for the function of such variation. We measured the ability of Chowchillas to recognise increasingly distant and dissimilar dialects using playback experiments in order to determine whether the 'relative threat' (small scale) and 'recognition' (large scale) hypotheses best explain our findings. Chowchillas recognised and approached all song dialects tested. However, they engaged in territorial song the quickest and most often upon hearing local song, with progressively fewer responses and increasing song latencies with increasingly distant and dissimilar dialects. Our results are best explained by the 'relative threat hypothesis', suggesting that the main function of dialects in this species is efficient territorial defence, with neighbours posing the greatest threat, explaining the declining singing response to more distant song. Our findings extend the generality of this pattern to tropical, group-living songbirds in which both males and females sing.