Social rank influences conspicuous behaviour of black-capped chickadees, Parus atricapillus

This study on black-capped chickadees, Parus atricapillus, assesses whether social rank influences the behaviour of chickadees in predator situations. Chickadee flocks were restricted to an area of an outdoor aviary containing no vegetative cover to ensure that all flock members were equally exposed to potential danger. Each flock was presented with two different predator stimuli and one control stimulus. All flocks formed stable, linear hierarchies and there appeared to be dominance-related risk-taking. After hearing hawk vocalizations, the chickadees uttered 'seeet' alarm calls and all birds became immobile (froze), but subordinate and middle-ranked birds (subdominants) typically were the first to break the freeze. No differences were found for the 'seeet' alarm call. When exposed to a hawk mount, all birds gave repeated 'chick-a-dee' calls but subdominants consistently gave the call sooner than their dominant flockmate. These results could not be attributed to a bird's sex. It is suggested that greater risk-taking by subdominants could be one mechanism causing higher rates of winter mortality among low-ranking birds in free-ranging chickadee flocks.