‘Chick-a-dee’ calls of Carolina chickadees convey information about degree of threat posed by avian predators

Individuals of many group-living species of birds respond to potential predators by giving alarm calls, and some species give different, spatially referential alarm calls in response to aerial predators (raptors in flight) and terrestrial predators (including perched raptors). In black-capped chickadees, Poecile atricapillus, characteristics of alarm calls also vary with predator size and level of perceived threat (Templeton et al. 2005, Science, 308, 1934–1937). We examined whether the terrestrial alarm calls of Carolina chickadees, P. carolinensis, show similar variation. In 2007 and 2008, we monitored responses of eight flocks of Carolina chickadees in Madison County, Kentucky, U.S.A. to mounts of different species of raptors that varied in size. Chickadees responded to the raptors by uttering ‘chick-a-dee’ calls with different numbers and types of notes. Larger, lower-threat predators (e.g. red-tailed hawk, Buteo jamaicensis) elicited calls with significantly more introductory ‘chick’ notes and fewer ‘dee’ notes, whereas smaller, higher-threat predators (e.g. eastern screech-owl, Megascops asio) elicited calls with few or no ‘chick’ notes and significantly more ‘dee’ notes. In addition, playback experiments revealed that a greater percentage of Carolina chickadees responded to playback of ‘chick-a-dee’ calls previously given in response to a small predator than during playback of calls given in response to a large predator. These results suggest that the ‘chick-a-dee’ alarm call is a graded signal that informs conspecifics about the presence and behaviour (i.e. perched) of a predator and the degree of threat posed by that predator.