News Spring 2010
Mark Nolen succesfully defends his PhD dissertation
Multi-species mobbing assemblages: behavioral asymmetries, vocal facilitation of social behavior, and the link of acoustic structure to function
Interspecific social groups are common in birds, mammals, and fishes. A diversity of socially-acquired information about group members’ behavior and predator presence from multiple species is an inevitable feature of such groups, if not the primary benefit of interspecific sociality. We used playback of predator vocalizations (Eastern screech-owl, Megascops asio) and playback of mobbing calls in the field to investigate vocal information flow in an avian flocking assemblage. Carolina chickadees (Poecile carolinensis) and tufted titmice (Baeolophus bicolor) are the primary sources of vocal public information related to the presence of dangerous aerial predators in mixed-species foraging flocks. White-breasted nuthatches (Sitta carolinensis) apparently follow chickadees and titmice to maintain this benefit. Our playback experiments conducted at Purdue’s Ross Biological Reserve and surrounding woodlands found that this asymmetry shifted towards a potentially more cooperative interaction during predator mobbing. Nuthatches displayed stronger mobbing behavior and initiated mobbing more often than other species. Chickadee calling rates and proximity to the predator model were positively correlated with those of heterspecifics, suggesting positive feedback between species’ mobbing intensities. We conducted a separate field playback experiment on individually radio-tagged tufted titmice in the reserve. Radio-tracking allowed us to locate male tufted titmice consistently without using their vocal behavior to find them – a key ability given that we were studying changes in vocal behavior. We hypothesized that titmice should distinguish between ‘Chick-A-Dee’ calls of the Carolina chickadee based on the amount of predation risk indicated by the number of D notes in the call. As predicted, titmice responded to playback of chickadee mobbing calls (calls containing a high number of D notes) by making more flights, increasing their vocal rates, and giving calls that contained more D notes. Titmice did not respond to playback of chickadee contact calls containing only a few D notes and indicative of low predation risk. Our results suggest that the tufted titmouse may be able to use the vocalizations of the Carolina chickadee – a frequent associate of the titmouse in mixed-species foraging flocks – to monitor chickadee behavior and the level of predation risk perceived by chickadees.