Repertoire matching between neighbouring song sparrows

A male song sparrow,Melospiza melodia, has a song repertoire of about eight or nine distinct song types, and he typically shares several of these song types with each of his several neighbours. In the prevailing theoretical view, the song types in a bird’s repertoire are interchangeable and multiple song types exist primarily to provide diversity. The present study was designed to test a contrary hypothesis concerning one particular context, counter-singing between neighbours. Specifically, the hypothesis was tested that song sparrows reply to the songs of particular neighbours with particular songs from their repertoire: they select a song type they share with that neighbour (‘repertoire matching’). In a field experiment, neighbour song was played to the subject from just inside the neighbour’s territory. Subjects responded with a song shared with that neighbour in 87.5% of trials (chance expectation for this sample is 42%). In control trials, where stranger song was presented from the same location, subjects responded with songs shared with the neighbour at that location in only 17% of the trials. It is suggested that ‘repertoire matching’ may be one advantage of a song learning strategy that produces song sharing between neighbours. ? 1996 The Association for the Study of Animal Behaviour Song is a signal by which a territorial male songbird advertises his breeding status and ‘posts’ his territory. Although a single species-specific song is sufficient for this purpose, in most songbird species the male possesses a repertoire of distinct song types. The function of song repertoires is a topic of considerable theoretical debate, but most theories of repertoires agree on one point: that the songs in the repertoire are interchangeable, functioning primarily to provide diversity (reviews in Krebs & Kroodsma 1980; Searcy & Andersson 1986; Catchpole 1987; Kroodsma 1988; Slater 1989; McGregor 1991). In the course of a long-term study of the song sparrow, Melospiza melodia, we have developed an alternative view of song repertoires that focuses on song sharing between neighbouring birds (Beecher et al. 1994). We have found that a young male song sparrow learns several songs from each of the older, established males in a particular area. Eventually the young bird attempts to set up his own territory close to his song ‘tutors’, often by defending a small ‘insertion’ territory among the larger territories and later expanding its boundaries. Because the young bird’s strategies of song learning and of territory establishment are correlated, he ends up sharing at least several song types with each of his future neighbours, both his ‘tutor’ neighbours, and other young birds who will have learned many of the same song types. We believe that sharing song types with his neighbours confers several advantages on the bird, and we focus here on one key advantage: it provides the bird with a mechanism for addressing or replying to a particular neighbour by singing a song type that he shares with that neighbour. We term this mechanism ‘repertoire matching’ because the reply song matches some song in the neighbour’s repertoire. In this paper we provide evidence for such a mechanism in song sparrows. One of the exceptions to the generalization that the bird uses the song types in his repertoire interchangeably is the case where the bird replies to a stimulus song by singing the same song type. This pattern of counter-singing has been referred Correspondence: M. D. Beecher, University of Washington, Box 351525, Seattle, WA 98195, U.S.A. (email: [email protected]). P. K. Stoddard is now at the Department of Biological Sciences, Florida International University, Miami, FL 33199, U.S.A. 0003–3472/96/040917+07 $18.00/0 ? 1996 The Association for the Study of Animal Behaviour