Multi-species mobbing assemblages: Behavioral asymmetries, vocal facilitation of social behavior, and the link of acoustic structure to function

Mark T Nolen, Purdue University

Abstract

Interspecific social groups are common in birds and mammals. A diversity of socially-acquired information about group members’ behavior and predator presence from multiple heterospecific sources 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 experiments 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. The motivational-structural rules hypothesis provided a framework to evaluate aggression and fearfulness of chickadees in relation to heterospecific behavior. During periods of high nuthatch calling rate, chickadee A notes indicated increased fearfulness and D notes indicated higher aggressiveness. When mobs contained more nuthatches, D notes indicated lower aggression. In mobs containing more chickadees, chickadee D notes were more aggressive. These data suggest that chickadee call structure may provide an insight into motivational tendencies across varying social and behavioral contexts. Finally, we found that the tufted titmouse can distinguish between chickadee vocalizations differing in the amount of risk indicated. Titmice responded to playback of chickadee mobbing calls with increased movements and increased vocal rates. During playback of mobbing calls, titmouse call note composition changed in different ways depending on behavioral context: the number of D notes decreased when titmice were actively flocking with other species but increased when titmice were alone and not flocking with heterospecifics. Titmice did not respond to chickadee contact calls indicative of low predation risk.

Degree

Ph.D.

Advisors

Lucas, Purdue University.

Subject Area

Ecology|Zoology

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