Frequency Shift in the Fee Bee Song of the Black-Capped Chickadee

The two-note, whistled fee bee of the Black-capped Chickadee (Parus atricapillus ) is considered one of the most simple and stereotyped of parid songs. During recent work on fee bee song recognition, the authors were surprised to record extensive frequency shifts in the fee bees of captive male chickadees. Here, they describe the pattern of frequency variation and its context, and examine Saunder's suggestion that males will shift frequency in response to hearing conspecifics. The analyses support earlier field observations of frequency shift in Black-capped Chickaddes, with two differences: each male had a single version of shifted song (lowered in frequency), and shifted songs were sung quite regularly. The authors conclude that variation within individuals in the frequency characteristics of fee bee songs is greater than recent sonagraphic work on lone males has implied.