Neighbor–stranger discrimination in Audubon's shearwater (Puffinus l. lherminieri) explained by a “real enemy” effect

Neighbor–stranger discrimination (NSD) occurs when animals respond with more aggression to strangers than to territorial neighbors. NSD has been reported in many species that defend multi-purpose territories for breeding and foraging, but it is rare among species that defend other types of territories. For birds that defend only their nest sites, there is no experimental evidence for NSD, and observational studies have provided mixed results. In a colony of Audubon's shearwaters (Puffinus l. lherminieri), I played back the calls of a neighbor and a stranger to males defending nest sites. Subjects responded with longer calls to playbacks of strangers than to those of neighbors. In shearwater colonies, strangers are often birds looking for future breeding sites. In contrast, there is no evidence that established breeders compete with their neighbors for any resources. Shearwaters should benefit from NSD because strangers represent a “real enemy” and established neighbors do not.