Seasonal Patterns of Song Structure Variation in a Suboscine Passerine

—Studies of song and its function in suboscine passerines are rare. We examined spatial and temporal variation in song structure in a wild population of Olive-sided Flycatchers (Contopus cooperi) and tested among hypotheses explaining this variation. Repeatable variation in song type was observed both within and among individuals. More than 10% of territorial males expressed atypical song types, i.e., permutations of sequential missing and repeated elements of the typical adult song. Atypical songs were predominantly expressed by unpaired males independent of habitat type. A small fraction of males sang atypical song through the middle of the breeding season, but all males sang only stereotypical adult song by the end of the season. These results suggest the expression of atypical songs reflect protracted vocal development rather than evolution of new song types, geographic variation in song structure, or an extensive song repertoire in Olive-sided Flycatchers. Received 17 November 2008. Accepted 18 March 2009. Suboscines comprise ,20% (1,151 species) of the Order Passeriformes (Sibley and Monroe 1990), but studies of bird song and its functions have focused almost exclusively on oscines. Suboscines are characteristically incapable of song learning (Kroodsma 1984, but see Saranthan et al. 2007). Substantial variation in suboscine song structure exists and likely has important implications for evolution of suboscine breeding systems. Some suboscine species exhibit geographic variation in song structure (Lindell 1998, Sedgwick 2001, Isler et al. 2005). Others exhibit a repertoire of song types (Craig 1943, Kroodsma 1985, Smith and Smith 1996, Lein 2007) and, even in species with a single song type (e.g., Vermillion Flycatcher, Pyrocephalus rubinus), males can vary the length of a single structural component resulting in ‘long’ or ‘short’ songs (Rı́os-Chelén and Garcia 2007). Males express different song types when associating with a female or interacting with competing neighboring males. Thus, as in oscine passerines, different aspects of song expression should vary in importance throughout the breeding season (Smith and Smith 1996, Rı́os-Chelén and Garcia 2007, Sexton et al. 2007). However, a paucity of studies examining variation in structure of suboscine song remains a barrier to a clearer understanding of the frequency and extent of variation in song among suboscine species. We investigated variation in song structure in the Olive-sided Flycatcher (Contopus cooperi) because reports of ‘alternate song types’ (Wright 1997 in Altman and Sallabanks 2000) suggest the existence of substantial extant variation, but the underlying source of this variation remains unclear. The Olive-sided Flycatcher is a migratory flycatcher in the family Tyrannidae (Sibley and Ahlquist 1990). The typical adult male song is a 3 Current address: W. K. Kellogg Biological Station, Michigan State University, 3700 East Gull Lake Drive, Hickory Corners, MI 49060, USA. 1 Avian Science Center, University of Montana, Missoula, MT 59812, USA. 2 Sonoran Desert Research Station, University of Arizona, Tucson, AZ 85721, USA. 4 Corresponding author; e-mail: [email protected] SHORT COMMUNICATIONS 815