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.