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Conspecific nest parasitism is much more common in birds that have self-feeding young than in those with parentally fed young. We review evidence for this pattern and suggest that it is produced by a fundamental dichotomy in selection pressure for defense against conspecific parasites. Species that feed their young should experience intense selection pressure for effective defense against parasitism, because their reproductive output is limited by post-hatching parental care. Species that have self-feeding young, however, experience only mild selection for parental defense against parasites, because their reproductive output is constrained chiefly by their ability to produce eggs. For such birds, parental care can be shared with little or no detriment to survival of young. Within both the self-feeders and the birds that feed their young the occurrence and intensity of conspecific nest parasitism is dictated primarily by the ability of parasitic females to find host nests. This pattern is particularly clear among waterfowl, where conspecific nest parasitism is frequent in ducks that nest in cavities, over water, or in high densities on islands. We review evidence that suggests greater conspecific nest parasitism for parentally fed species that nest in colonies than parentally fed species with dispersed nests. We also make predictions about the distribution of conspecific nest parasitism in nonavian groups.