Alarm calls and mobbing behavior associated with predation risk in nesting Black Terns (Chlidonias niger).

Abstract

Predation on chicks by large birds (e.g. gulls) and other mammals has long been implicated as a major selective force in the evolution of parental care characteristics in birds. Calling and mobbing potential predators are often useful but costly tactics for driving the predator away: frequent mobbings may decrease producitivity by reducing time spent incubating, brooding, and guarding eggs and chicks. Growing evidence suggests birds can discriminate among potential predators and selectively respond to them. I analyzed predator response selectivity among a colony of nesting Black Terns (Chlidonias niger) to aerial (i.e. gulls) versus terrestrial (i.e. humans) predators. Aerial predators illicited more intensive strengths of defense than terrestrial predators, as demonstrated by the terns' (1) larger mob sizes (p<0.001) and (2) greater call rates (p<0.001). Distances of predators which illicited mobbings, however, were greater for terrestrial predators than for aerial predators (p<0.001). These results supported the hypothesis that Black Terns can selectively discriminate and respond to potential predators.