Lack of alarm calls in a gregarious bird: models and videos of predators prompt alarm responses but no alarm calls by zebra finches

Abstract

Many vertebrates use vocalizations to communicate about the presence of predators, and some encode information about predator type or behavior. A fast-approaching predator typically elicits a “flee alarm call,” prompting others to escape to safety. In a field experiment, we presented gliding models of a predatory bird to several species representing four families of passerine, including our model species, the zebra finch (Taeniopygia guttata). All families presented with the glider gave a distinct call on at least one occasion, apart from the zebra finch, for which no specific alarm call was recorded. Following on from this unexpected result, we conducted an experiment in which we exposed captive zebra finches to video of a looming raptor. Results of the captive study showed that birds responded to the looming raptor with escape behavior and responded to less threatening stimuli with orienting or startle behavior. Despite this anti-predator response, birds did not give any distinct alarm call, and the distance calls of both males and females did not differ in structure or rate of delivery after exposure to a stimulus. Zebra finches are one of the most studied birds in the world, are gregarious, and have a rich vocal repertoire, yet their alarm communication has not been investigated experimentally. Our results are consistent with the hypothesis that zebra finches lack a flee alarm call and appear not to signal about immediate danger through a change in calling rate. Many animals emit alarm calls when faced with a threatening event in order to communicate with nearby group members. Threatening events can be simulated with models or by presenting a video of a looming stimulus on a screen. In separate studies, we presented gliding models and computer animations of a hawk to zebra finches, a bird species used in studies around the world, in order to test if they gave alarm calls to warn others of approaching danger. Although birds fled in response to the simulated predators, they did not emit a distinct alarm call. The birds also did not change their rate of calling or the acoustic structure of their distance calls. Surprisingly for a social and highly vocal species, the birds appear to lack alarm calls warning flockmates of immediate danger.