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.
Our study was conducted at the Queen's University Biological Station at Lake Opinicon, Ontario, about 50 km north of Kingston. Recordings were made from 1 April to 30 June 1989, and from 19 April to 8 June 1990. Subjects were color banded at winter feeding stations before the study began. Territories were mapped using song playbacks (Falls 1981) and/or observations of territorial interactions.