Social behavior and long-distance vocal communication in eastern American crows.

Abstract

To examine the relationship between social organization and vocal behavior, I studied wild family groups of eastern American Crows, Corvus brachyrhynchos brachyrhynchos in Ann Arbor, Michigan. Some individuals tagged as nestlings acted as helpers at the nest in subsequent years. Helpers did not seem to gain inclusive fitness benefits, at least in the short term. Instead, ecological variables unrelated to group size were more important in nest success. Helpers could have been gaining parenting experience. Crows were partially migratory and group-territorial year round, though some individuals moved in and out of non-territorial summer flocks and roosting winter flocks. Males often remained with their parents longer than females, who may have been more likely to migrate and disperse. Caws recorded from wild and captive crows had a call-like structure, particularly in being harsh and graded. The repetitive, rhythmic, and complex organization of these caws into series, however, resembled vocalizations used by other songbirds in mating competition contexts. Some caw series types appeared to serve as general alarm signals, while unstructured cawing was clearly associated with predator mobbing. However, some caw series types are strongly associated with song-like contexts and one caw series type, doubled short caws, tracked seasonal changes in territorial bout frequency. Crows responded more strongly to playback of doubled cadence than to regular cadence, indicating a role for rhythm in repertoire size determination and supporting a hypothesis that doubled short caws are more territorial, or song-like, in function. Crows also responded slightly differently to caw series of similar cadence but different caw durations. Breeding pairs of crows responded more strongly to playback of strangers than did larger family groups. This result is consistent with a helper-reduced territoriality hypothesis: the presence of helpers lowers territorial response levels because they can usually passively deter intrusions or because the group's odds of success in a fight are great even if intruders engage them. Territoriality in a changing social milieu may provide selection pressure for song-like ritualization and elaboration of otherwise call-like signals.