SONG CORRELATES WITH SOCIAL-CONTEXT, TESTOSTERONE AND BODY CONDITION IN MALE BARN SWALLOWS

Bird song, like many other male secondary sexual characters, may have evolved as intra- or inter-sexual signals of male phenotypic quality. The hypotheses that song rate and song features reflect androgen level s and body condition, qualities useful in male-male competition, and t hat they are also influenced by social context, was tested for the fir st time in the present correlational study. The relationships between song rate and 14 variables describing song structure, respectively, an d absolute plasma testosterone levels, body mass, body condition, numb er of neighbouring males and distances between nest sites in male barn swallows, Hirundo rustica, were analysed. Song rate was not correlate d with any of the song features nor with male or social context charac teristics. By contrast, a harsh song syllable, the 'rattle', was posit ively related to plasma testosterone levels, and its peak amplitude fr equency varied inversely with male body mass and condition. In additio n, eight features of song varied according to the social environment o f each male. In particular, males sang longer and more varied songs wh en they had few or no neighbours, whereas males in highly competitive contexts uttered short songs, interrupted them more frequently, and em phasized the rattle. Neighbouring males also sang more similar songs t han distant males, and this resulted in matched countersinging. The qu ality of song output therefore reflects aspects of male competitive po tential, and relationships between song structure and social context s uggest that some features, such as the rattle, might have originally e volved to serve in male-male interactions; a female preference may hav e further promoted song evolution leading to complex syllable repertoi res. (C) 1997 The Association for the Study of Animal Behaviour.