Soft Song in Song Sparrows: Acoustic Structure and Implications for Signal Function

Male songbirds are known for their conspicuous broadcast songs. In most songbird species, broadcast songs are produced at high amplitudes such that their active space (the distance from a sound source over which a signal is detectable) extends well beyond territorial boundaries (Brenowitz 1982; Wiley & Richards 1982; Dabelsteen et al. 1993; Holland et al. 1998; Naguib & Wiley 2001). In addition to broadcast songs, songs of low amplitude, variously termed ‘quiet song’, ‘twitter song’, ‘whisper song’ or ‘soft song,’ have been noted for some songbird species. Examples include the European blackbird Terdus merula (Dabelsteen 1984; Dabelsteen & Pedersen 1990), the European robin Erithacus rubecula (Lack 1965; Dabelsteen et al. 1997), the dunnock Prunella modularis (Snow 1988), the alpine accentor Prunella collaris (Langmore et al. 1996), the dark-eyed junco Junco hyemalis (Titus 1998), and the whitethroat Sylvia communis (Balsby 2000; Balsby & Dabelsteen 2002). Morton (2000) notes personal observations of ‘whisper song’ in 24 species of North American passerines. Soft song has not been closely studied in most species; hence the contexts in which soft songs are produced, as well as their signal value, remain poorly understood. The best studied cases are lowamplitude ‘twitter song’ in the blackbird (Dabelsteen 1984; Dabelsteen & Pedersen 1990), and ‘short-range song’ in the dark-eyed junco (Titus Correspondence Rindy C. Anderson, Department of Biology, Duke University, Durham, NC, USA. E-mail: [email protected]