DOES NEST DEFENSE DETER OR FACILITATE COWBIRD PARASITISM - A TEST OF THE NESTING-CUE HYPOTHESIS

The nesting-cue hypothesis poses that avian brood parasites use nest-d efence responses directed toward them be hosts as cues to locate nests to parasitize. Hosts that respond more intensely to brood parasites s hould provide more cues about nest location than those hosts giving lo wer intensity responses. Thus, the nesting-cue hypothesis predicts tha t within a species parasitized nest owners should respond more intense ly than unparasitized nest owners to combirds perched near acid away f rom nests. This assumes that hosts respond to cowbirds when they are e ncountered away from the nest and that host responses gradually increa se in intensity as the cowbird nears the nest. The nesting-cue hypothe sis, its assumptions and prediction mere tested using six host species of the brown-headed cowbird (Molothrus ater, Icterinae). We presented a female cowbird model at three distances from host nests and compare d the responses elicited. All species responded to the cowbird at all distances, which supports the first assumption. Some of the rarely eli cited responses (e.g. contacts) and the proximity of the host to the m odel varied significantly with distance, which suggests that cowbirds could use nest defence by the host as cues to the location of an activ e nest. However, parasitized nest owners did not respond more intensel y than unparasitized nest owners to the cowbird positioned at any of t he distances from tile nest, which does not support the nesting-cue hy pothesis itself. Further considerations are discussed that suggest tha t nest defence is not likely to be used as a nest-location cue.