Experimental manipulation of mixed-species flocks reveals heterospecific audience effects on calling

Abstract

Animal signallers are subject to audience effects when they alter communication due to changes in the presence or characteristics of receivers. Studies aimed at understanding audience effects have typically examined effects of conspecific audiences on signaller communication. Less work has focused on heterospecific audiences, which present an important avenue of research for species that participate in mixed-species groups. Here we experimentally tested mixed-species flocks of Carolina chickadees, Poecile carolinensis, and tufted titmice, Baeolophus bicolor, for conspecific and heterospecific audience effects. Birds were trapped from naturally occurring flocks and held in seminatural outdoor aviaries, where we recorded calling. We found that chickadees and titmice were sensitive to the number of conspecifics in flocks when communicating via ‘chick-a-dee’ calls, which are social cohesion calls produced by both species. Chickadees also were sensitive to the number of titmice in flocks, but chick-a-dee calling behaviour in titmice did not differ with regard to the number of chickadees in flocks. Furthermore, when subject to playbacks of simulated risk, chickadees and titmice produced more chick-a-dee calls when more titmice were in a flock. After these playbacks, chickadees produced fewer chick-a-dee calls with increasing numbers of conspecifics in flocks, whereas titmice produced more chick-a-dee calls with increasing numbers of conspecifics in flocks. These results suggest that chickadees and titmice are sensitive to social factors within their mixed-species flocks when communicating, and that chickadees appear more sensitive to heterospecific presence than do titmice. We suggest this is due to the dominance status of the species in these flocks, where titmice are typically dominant over chickadees.