The alarm call system of breeding Brown Thornbills (Acanthiza pusilla): self-defence or nest defence?

Abstract

Many species produce alarm calls during predator encounters that are directed at either conspecifics or the predator. Although many studies have investigated alarm calls in foraging individuals, antipredator communication during reproduction is rarely studied. In birds, where nest predation is a key cause of reproductive failure, some species have evolved sophisticated alarm calls to inform nestlings of danger from nest predators. However, different predator species differ in their primary prey type (brood predators, predators of adults), and accordingly, birds could have evolved different alarm calls depending on who is in danger. We experimentally investigated parental antipredator communication in Brown Thornbills (Acanthiza pusilla), where more than 50 % of all nests are depredated. We presented breeding pairs the models of a brood predator, a predator of adult birds and a harmless control species. Parents gave two different alarm calls (short and intense tzzt calls), which were not predator-specific, but responded to the predator of adults with more alarm calls than for the brood predator. Parental alarm calls did not affect nestling begging levels, which were influenced only by parental feeding rate. Our results suggest that alarm calls of breeding Brown Thornbills are mainly used as a form of self-defence by the individuals of the breeding pair rather than being directed at the nestlings. This fits with the predictions of life-history theory that parents in long-lived species are more concerned about their own survival.