Neotropical Birds Respond Innately to Unfamiliar Acoustic Signals.

AbstractMany animals respond to heterospecific signals that indicate the presence of food or predators. Although the benefits of responding are clear, the behavioral and cognitive mechanisms underlying responses are not. Whether responses are learned, innate, or an epiphenomenon created by following other species as they respond to signals remains unknown because most studies have involved respondents that are sympatric with their heterospecific signalers and that have therefore had opportunities to learn their signals. In this study, we tested the mechanisms underlying avian responses to heterospecific chick-a-dee calls. All North American parids produce chick-a-dee calls in response to arousing stimuli, such as food and predators, and diverse species respond by approaching the caller and consuming the food or mobbing the predator. We broadcast chick-a-dee calls plus two control stimuli in Costa Rica, Colombia, and Brazil, where no parids ever occur. We conducted our trials in the winter, when Neotropical migrants that might be familiar with chick-a-dee calls were present, and in the temperate breeding season, when migrants were absent. Across 138 trials, 38 resident species from 14 families and four orders responded to chick-a-dee calls by approaching to within 5 m of the playback speaker. A phylogenetic logistic regression showed that whether a species responded was not significantly associated with the species' mean body mass or the structural similarity between its calls and chick-a-dee calls. Residents were significantly more likely to approach chick-a-dee calls than either control stimulus. This pattern was unaffected by the presence of migrants, thus demonstrating that the observed responses are innate. Our study shows that learning cannot fully explain responses to heterospecific chick-a-dee calls and that structural features distinguishing these calls from other vocalizations are important.