Nest desertion by blue-gray gnatcatchers in association with brown- headed cowbird parasitism

The blue-gray gnatcatcher, Polioptila caerulea, is one of the smallest regular hosts of the brown-headed cowbird, Molothrus ater. Gnatcatchers typically raise none of their own young when parasitized, yet anti-parasite behaviours have not been reported. A blue-gray gnatcatcher population in which pairs often desert nests when parasitized by cowbirds was studied to examine the success of a desertion response and to determine factors that stimulate desertion. Of 83 gnatcatcher nests, 76% were parasitized; of nests parasitized during the egg-laying stage, 45% were deserted. Desertion occurred independently of the timing of the parasitism event during the breeding season and the number of cowbird eggs laid per nest. All cowbird eggs laid after the completion of the gnatcatcher's egg-laying stage were accepted. Parasitism rates were lower late in the breeding season, and nesting success of unparasitized nests was higher. Pairs that deserted often obtained cowbird-free clutches, allowing them to raise gnatcatcher young. Nest desertion appears to be a means of avoiding parasitism and increasing fitness. The nest desertion response was inconsistent, but it cannot be explained by the adaptiveness of acceptance or by the possibility that blue- gray gnatcatchers are in transition between being acceptors or rejectors of cowbird parasitism. Nest desertion by blue-gray gnatcatchers appears to be a generalized response to a disturbance at the nest induced by cowbirds. The cue that elicits the desertion response remains unclear.