Shift of song frequencies in response to masking tones

Ambient noise can interfere with signal transmission and detection across many taxa and modalities. Evidence suggests that, over time, signals evolve to minimize interference from ambient noise and other signalling animals. Less well studied is the possibility of short-term behavioural responses to transient ambient noise, in which animals actively adjust signal parameters to recover signalling efficacy. Here we test animals' capacity to adjust vocal signal parameters in the face of transient acoustic interference. In field trials we monitored the songs of territorial male black-capped chickadees, Poecile atricapillus, determined the frequencies of their 'fee-bee' songs, and broadcast tones to closely mask subjects' 'bee' notes. We also presented control nonmasking tones of 5 kHz, well above birds' song frequencies. Our main finding was that males responded to masking tones by shifting song frequencies after an average of 66.4 s from tone onset, whereas frequency shifts in the presence of nonmasking tones occurred only after an average of 95.8 s. The quicker shift in frequencies in the face of masking noise provides new evidence for vocal behavioural plasticity, and further reveals how behavioural plasticity together with evolutionary adaptations can minimize the detrimental effects of ambient noise on communication. (C) 2012 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.