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. ? 1996 The Association for the Study of Animal Behaviour Avian brood parasites often reduce the fitness of their hosts by lowering reproductive success (Payne 1977; May & Robinson 1985). Selection theory predicts that hosts should evolve behaviour patterns that maximize fitness by minimizing the effects of brood parasitism (Rothstein 1975a; Davies & Brooke 1989a, b). Ejection of the parasite egg and desertion of parasitized nests are potential examples of these behaviour patterns. The intensity of selection for such behaviour patterns should be highest in populations that are frequently parasitized and suffer large reductions in nesting productivity when parasitized (Rothstein 1975a; Davies & Brooke 1989b). Egg ejection, i.e. the removal of the cowbird egg either by grasping the egg in the beak or by puncturing and then lifting the egg, is a common response to parasitism by many larger host species (Rothstein 1975a). Nest desertion is often a more common response among smaller hosts (Friedmann 1963; Graham 1988), perhaps because small hosts lack the mandible size or strength to eject the parasite egg. Unlike eggejecting species, which consistently give the ejection response to cowbird parasitism (Rothstein 1975b), nest-deserting species are usually inconsistent in their desertion response to cowbird eggs (Graham 1988; Hill & Sealy 1994), raising questions of what proximate cues stimulate this response. The blue-gray gnatcatcher, Polioptila caerulea, is a widely distributed, woodland-dwelling songbird that is a well-known host of the brood parasitic brown-headed cowbird, Molothrus ater (Bent 1949; Friedmann 1963, 1971). It is also among the smallest of regularly-parasitized brown-headed cowbird hosts (Bent 1949) and Correspondence: C. B. Goguen, Department of Wildlife Ecology, University of Wisconsin, 226 Russell Labs, Madison, WI 53706, U.S.A. (email: goguen@calshp. cals.wisc.edu). N. E. Mathews is now at the same address. 0003–3472/96/090613+07 $18.00/0 ? 1996 The Association for the Study of Animal Behaviour