Variation in chick-a-dee call sequences, not in the fine structure of chick-a-dee calls, influences mobbing behaviour in mixed-species flocks

When animals vocalize under the threat of predation, variation in the structure of calls can play a vital role in survival. The chick-a-dee calls of chickadees and titmice provide a model system for studying communication in such contexts. In previous studies, birds’ responses to chick-a-dee calls covaried with call structure, but also with unmeasured and correlated parameters of the calling sequence, including duty cycle (the proportion of the calling sequence when a signal was present). In this study, we exposed flocks of Black-capped Chickadees (Poecile atricapillus) and heterospecific birds to playback of chick-a-dee calls and taxidermic models of predators. We quantified birds’ responses to variation in number of D-notes and duty cycle of the signalling sequence. Chickadees and heterospecific birds responded more intensely to high-duty-cycle treatments, and equally to treatments where duty cycle was held constant and the number of D-notes varied. Although our study does not disenta...