Research Papers A Test of the Effect of Male Song on Female Nesting Behaviour in the Serin (Serinus serinus): a Field Playback Experiment Paulo G. Mota & Violaine Depraz Laboratory of Ethology, Institute of Environment and Life, Department of Anthropology, University of Coimbra, Coimbra, Portugal Abstract It is well established, through laboratory experiments, that male song in birds can stimulate female reproductive activity, affecting their behaviour and phys- iology, such as follicular growth, nest building and egg-laying. However no clear demonstration has yet been provided that this effect works under natural conditions. Previous work in natural populations of serins showed that female nest-building behaviour correlated with male singing time. Furthermore male serin song peaked exactly in the day that rapid follicular growth was estimated to start in females, suggesting that in this species song may also serve to stimulate the female’s reproductive development. Direct causal evidence, however, was lacking. We conducted field playback experiments to investigate how song can influence female nesting activity during nest building. Our results show that females who listened daily to playbacks of serin songs, during the nest-building stage, spent more time nest building than females that were not exposed to additional songs. Moreover, the singing behaviour of the mated males was not affected by the playbacks, suggesting that the song playback treatment had a direct positive effect on female nesting behaviour. Correspondence: P. G. Mota, Laboratory of Ethology, Institute of Environment and Life, Department of Anthropology, University of Coimbra, 3000-056 Coimbra, Portugal. E-mail: pgmota@ci.uc.pt Introduction It is generally accepted that male bird song has three main functions that work in the context of sexual selection. Bird song is used to repel rival males (territorial function), to attract and entice females (mate attraction), and to stimulate female reproductive activity (mate stimulation) (Kroodsma & Byers 1991; Baptista & Gaunt 1994; Catchpole & Slater 1995). These functions are not mutually exclusive, although one may be more important than the others, Ethology 110, 841—850 (2004) Ó 2004 Blackwell Verlag, Berlin ISSN 0179–1613 U. S. Copyright Clearance Center Code Statement: 0179-1613/2004/11011–841/$15.00/0 www.blackwell-synergy.com