DESERTION OF NESTS PARASITIZED BY COWBIRDS - HAVE CLAY-COLORED SPARROWS EVOLVED AN ANTIPARASITE DEFENSE

Clay-coloured sparrows, Spizella pallida, have often been reported des erting nests parasitized by brown-headed cowbirds, Molothrus ater. Spa rrow nests were experimentally parasitized and manipulated to determin e whether desertion is an anti-parasite defence. Responses to experime ntal parasitism with and without the presence of a cowbird model, the addition of broken eggs to the nest, changes in clutch composition (co mbination of sparrow and cowbird eggs), clutch reductions, and human n est visitation were recorded. Only clutch reduction elicited nest dese rtion. Because such reductions can occur in the absence of brood paras itism, desertion apparently has not evolved as an anti-parasite defenc e in this species. Desertion of naturally parasitized nests appears to be a response to host egg removal by female cowbirds, rather than to parasitism per se. The absence of anti-parasite defences in clay-colou red sparrows is not adequately explained by the equilibrium hypothesis and, contrary to an earlier suggestion, clay-coloured sparrows are no t in transition between being an acceptor and rejector species of cowb ird parasitism. More attention needs to be paid to the host egg remova l aspect of cowbird parasitism.