Differential responses to male song repertoires in female songbirds implanted with oestradiol

Abstract

Considerable debate surrounds the functional and evolutionary significance of the complex song repertoires found in male passerine birds^1–3. A comparative study on European warblers of the genus Acrocephalus⁴ has suggested that sexual selection is the driving force behind the evolution of the extraordinary song complexity in this group. One species in particular, the sedge warbler Acrocephalus schoenobaenus, sings long, variable, continuous songs constructed from a repertoire of discrete building blocks called syllables, revealed by sonagraphic analysis⁵. Although a strong inverse correlation between male repertoire size and pairing date was obtained from this wild population of sedge warblers, it was difficult to control adequately for confounding variables of male or territory quality which may also influence female choice at the time of pairing. It has been demonstrated recently that female songbirds implanted with oestradiol respond to playback of recorded song in laboratory conditions^6–10; the resulting displays have been used as an index of response to a variety of natural and experimental songs. Here we have used both natural recordings from an earlier field study and experimental songs to demonstrate that implanted female sedge warblers respond significantly more to playback of higher repertoire sizes in standard laboratory conditions.