Stimuli Eliciting Distress Calls in Adult Passerines and Response of Predators and Birds to Their Broadcast
This study examined the response of birds and captive predators to the broadcast of distress calls and the effect of different stimuli on the elicitation of these calls. In doing so, this study tested two hypotheses about why adult passerines should distress call when physically constrained: the calls are designed 1) to attract attention, or 2) to startle the predator into releasing the caller. Birds often responded to both interspecific and intraspecific distress calls by approaching the sound source, but they rarely mobbed or engaged in any behavior that would aid the caller in escaping. The playback of a distress call had little effect on most captive opossums (Didelphis marsupialis) and raccoons (Procyon lotor) which were attacking a caged starling (Sturnus vulgaris). However, distress calls startled one opossum and two raccoons and provoked two other raccoons into a more severe attack. Birds only distress. called when physically constrained. All passerine species that were tested, except brown-headed cowbirds (Molothrus ater), emitted distress calls, but in no species did every individual call. Distress calls usually were of short duration, interrupted by periods of silence, and paired with struggling behavior. Birds were more likely to distress call when held by the limbs rather than the body or neck, when moved, or when viewing a rapidly approaching object. These results indicate that one function of distress calls for most passerines is to startle the predator, but that other functions also are likely. My results also support the hypothesis that birds approach a distress caller to acquire information about the predator that has captured the caller.