Asymmetries in mobbing behaviour and correlated intensity during predator mobbing by nuthatches, chickadees and titmice

In mixed-species foraging flocks of forest birds, one or a few nuclear species frequently produce alarm calls and are followed by other species in the flock. We tested the hypothesis that similar asymmetries may exist in a second interspecific social context, multispecies mobbing behaviour. We examined mobbing behaviour evoked by an eastern screech-owl, Megascops asio, model and playback in two nuclear species, Carolina chickadees, Poecile carolinensis, and tufted titmice, Baeolophus bicolor, and in a species that follows them, the white-breasted nuthatch, Sitta carolinensis. Asymmetries in mobbing were not the same as those in mixed-species flocks. Nuthatches and chickadees mobbed with greater frequency and intensity compared to titmice, which remained at greater distances from the owl model and vocalized less frequently. We also tested for the existence and nature of potential interspecific vocal information flow during mobbing. Chickadee and nuthatch calling rates were positively correlated, as were chickadee and titmouse calling rates. Nuthatches and titmice rarely mobbed simultaneously. These results suggest the existence of positive feedback among species' mobbing intensity during a multispecies mobbing association as opposed to heterospecific vocal interference or a lack of heterospecific influence. However, randomization simulations showed that this positive feedback was not driven by a particular ‘nuclear’ species during mobbing, suggesting that the correlations may result from a mutually interdependent escalation of mobbing intensity.