Song degradation and estimation of acoustic distance in Black-capped Chickadees (Parus atricapillus)

Black-capped Chickadees (Parus atricapillus) sing a two-note song termed fee bee. Frequency descends within the fee note and between the fee and bee notes. Within-song frequency ratios are highly invariant, while among songs, absolute frequency is variable. We conducted sound-transmission studies to analyze how Black-capped Chickadee songs degrade in different native habitats. We then subjected colour-banded territorial males to playback of undegraded and degraded songs to see if they could estimate the distance to a conspecific singer by means of degradation cues. In transmission studies conducted in an open field and a dense deciduous forest, significant effects of reverberation and amplitude modulation were measured for songs at three different frequencies. A new technique, cross-correlation of song spectrograms, allowed for a measure of holistic song degradation, which varied significantly over distance in open versus forested habitat. Polynomial regressions for the effects of reverberation, amplitude modulation, and cross-correlation measures of general degradation revealed different trends in open versus forested habitat, regardless of absolute frequency. Male Black-capped Chickadees did not differ in their response to playback of undegraded and degraded songs when the two types were broadcast from the same position in the territory at the same amplitude. We suggest that heterogeneity of Black-capped Chickadee native habitat may preclude the use of song degradation as a reliable distance cue.