Call for Your Life: Acoustic Structure and Age-Sex Differences in Distress Calls of Red-Necked Nightjars

Abstract

Predatory interactions result in strong selection pressures acting on multiple aspects of animal behaviour. Anti-predator strategies are therefore common in most animals, typically signalling at various stages of a predation event. Many species of caprimulgids perform conspicuous anti-predator displays, including stereotyped behaviours and vocal signals. Here, we described distress calls of red-necked nightjars (Caprimulgus ruficollis), produced when birds are trapped and unable to escape during a predatory interaction. Distress calls are harsh, low-frequency guttural vocalisations with irregular amplitude modulations. The age and sex of birds partially explained the acoustic variation observed, whereas size-related morphological features were poor predictors of the acoustic structure. Age-sex differences in distress calls may respond to physiological constraints associated with sexual dimorphism and/or developmental variation. Alternatively, directional selection associated with differential predation risk on each age-sex class may have resulted in the observed differences in distress calls. The extremely deep voice and the harsh quality of distress calls fit the structure of aggressive signals and may resemble those produced by a larger animal. We propose that these calls serve as a last resort strategy to reduce post-encounter risk of predation, either as a startling effect to facilitate escape or to attract other predators that could intimidate the captor.