Frequency variation in songs of black-capped chickadees (Parus atricapillus)

-Recordings of dawn singing by male Black-capped Chickadees (Parus atricapillus) show that each individual sings its fee-bee song at a wide range of frequencies. Males tend to repeat songs at a given frequency but on average after every 41 ? SE of 8.8 songs a male shifts the frequency of its song by a statistically significant amount (?80 Hz). During any given morning, males may appear to shift among a limited set of discrete frequencies, but over longer time periods intermediate frequencies also are sung. These results suggest that chickadees can vary the frequency of their song more or less continuously over the species' frequency range. When songs of one of three widely spaced frequencies (recorded in previous years) were played back, males replied with songs that had approximately the same frequency as the playback song. Thus, frequency shifting appears, at least in part, to be a form of song matching. These results add to a growing body of evidence that some species with single-song repertoires have evolved effective matching strategies through manipulation of the frequency of their song. Received 14 June 1991, accepted 20 January 1992. THE MAJORITY of songbirds possess multisong repertoires, in which song types can be easily distinguished from each other on the basis of several frequency and temporal features. There has been considerable effort to understand the interand intrasexual selective pressures favoring the evolution of multisong repertoires (e.g. Kroodsma and Miller 1982, Searcy and Andersson 1986). In contrast, the conditions that predispose species to maintain single-song repertoires have received comparatively little attention. By single-song repertoires, we mean those in which songs may vary in a graded fashion (e.g. longer or shorter), but cannot be divided into groups by discontinuities in any single parameter or combination of parameters. Species with single-song repertoires may show little variation in song structure because of some advantage to stereotypy, for example, because low variation within and between males in a given area aids the recognition of dialects (e.g. Baker and Cunningham 1985). Alternatively, relatively subtle variations in song structure may serve functions that, in a repertoire species, would be served by abrupt changes in song type. For example, variation in the frequency of the songs of Kentucky Warblers (Opornis formosus; Morton and Young 1986) and Harris' Sparrows (Zonotrichia querula; Shackleton et al. 1991) allow males to match the frequency of each other's songs as a signal in territorial contests. In species with song types, this function is served by males matching each oth-