Referential signaling in a communally breeding bird.

Referential signaling, a complex form of communication in which specific signals are associated with external referents, was once thought to be limited to primates. Recent research has documented referential signaling in several other cooperative taxa, predominantly in kin-based societies. Here, we show that greater anis, communally nesting birds that breed in nonkin groups, give one type of alarm call in response to aerial threats (flying raptors) and another to more general threats (nonaerial predators). Observational data show that anis give these calls in response to different classes of threats, and playback experiments in the field confirmed that the alarm calls alone are sufficient to elicit appropriate behavioral responses even in the absence of an actual threat. Genetic data on a subset of groups confirmed that breeding groups are composed of nonkin, suggesting that referential alarm calls are often given in situations when no genetic relatives are present. These results suggest that complex referential communication can occur in social groups composed of nonrelatives, despite the absence of kin-selected fitness benefits.