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Clay-coloured sparrows, Spizella pallida, have often been reported deserting nests parasitized by brown-headed cowbirds, Molothrus ater. Sparrow nests were experimentally parasitized and manipulated to determine whether desertion is an anti-parasite defence. Responses to experimental parasitism with and without the presence of a cowbird model, the addition of broken eggs to the nest, changes in clutch composition (combination of sparrow and cowbird eggs), clutch reductions, and human nest visitation were recorded. Only clutch reduction elicited nest desertion. Because such reductions can occur in the absence of brood parasitism, desertion apparently has not evolved as an anti-parasite defence 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-coloured sparrows is not adequately explained by the equilibrium hypothesis and, contrary to an earlier suggestion, clay-coloured sparrows are not in transition between being an acceptor and rejector species of cowbird parasitism. More attention needs to be paid to the host egg removal aspect of cowbird parasitism.