Parent-offspring communication under the risk of predation in the white-browed scrubwren

Abstract

Acoustic signaling is in important way that animals communicate, but the features that enhance the detectability of signals by receivers may also enhance detectability by eavesdropping predators. Parent-offspring communication in birds offers a superb window into how animals communicate under the risk of predation, as most species rely heavily upon vocal signaling and are particularly vulnerable to predation during the reproductive period. This thesis describes a series of experiments on nesting white-browed scrubwrens, Sericornis frontalis, focusing on three main questions: 1) Do nestling calls attract predators? Chapter 2 presents the first realistic test of the cost of nestling begging on predation risk at active nests attended by parents. Nestling vocalizations attracted predators, and this risk was highest for the hungriest and therefore noisiest broods (Haff & Magrath 2011, Biology Letters). 2) How do parents manage the trade-off between warning young of danger and betraying nest location to predators? Chapter 3 shows that parents were more likely to warn noisier compared to quieter nestlings when an eavesdropping predator was near the nest, demonstrating that parents take nestling conspicuousness into account in a decision rule likely to reduce the risk of nest predation. This work is in preparation for publication. 3) How do young reduce predation risk independently of parents? Chapter 4 shows that nestlings respond with silence to very specific features of the sound made by a predator walking near the nest. They are therefore able to reduce risk independently of parents, while not suppressing calling unnecessarily (Haff & Magrath 2010, Animal Behaviour). Chapter 5 shows that young nestlings respond to heterospecific mobbing alarm calls that are structurally similar to parental mobbing alarm calls, while older nestlings can respond appropriately to heterospecific alarm calls that are not similar to those of conspecifics. Nestlings therefore gain information about danger from the signals of other species, potentially through both innate recognition and learning. This work has been submitted for publication. Chapter 6 focuses on the development of fledgling responses to heterospecific aerial alarm calls, which are given to flying predators and are relevant to fledglings but not nestlings. Fledglings just out of the nest did not suppress calling after playback of heterospecific aerial alarm calls, but most did so only two weeks later and all responded by the time they were five-weeks old. Furthermore, young responded at an earlier age on territories on which heterospecific were more common, suggesting that learning was important in the recognition of heterospecific aerial alarm calls. Overall, this thesis illustrates that the behaviour of both parents and young can strongly affect the risk posed by nest predators, and helps to advance an understanding of parent-offspring communication under the risk of predation. An appendix to the thesis includes a broad review of this topic, co-authored by myself, R. D. Magrath, A. G. Horn & M. L. Leonard (Magrath et al. 2010, Advances in the Study of Behavior). -- provided by Candidate.