Responses by breeding birds to heterospecific song and mobbing call playbacks under varying predation risk

Abstract

We conducted a playback experiment to study the effect of predation risk on the attraction of forest passerines to heterospecific territory songs and mobbing calls during the breeding season. The experiment consisted of five treatments, which were played at different distances from the nests of sparrowhawks, Accipiter nisus: the territory song and mobbing calls of the willow tit, Parus montanus, and of the redwing, Turdus iliacus, with classical music as a control. Song may indicate the absence of predators whereas mobbing indicates a potential danger. We assumed that predation risk increases with decreasing distance to a hawk's nest. Therefore, we predicted that the correlation between the number of species attracted and increasing predation risk would be positive for song and negative for mobbing calls. We also examined the effect of body mass on the species' responses. In general, song and mobbing calls attracted more species than the control. For both willow tit and redwing treatments, mobbing calls elicited the strongest response and these attracted more species the lower the predation risk. Attraction to territory songs, however, did not vary in relation to predation risk. Small bird species (<20g) showed increasing attraction to mobbing calls with decreasing risk, but large bird species did not. Out results suggest that birds exploit heterospecific signals in their decision making but under the constraints of predation risk.