Comment: Why is conspecific nest parasitism more frequent in waterfowl than in other birds?

Rohwer and Freeman (F. C. Rohwer and S. Freeman. 1989. Can. J. Zool. 67: 239–253) have suggested that conspecific nest parasitism is rare in species with parentally fed young because high costs to hosts result in strong selection for defenses against parasitism. In contrast, conspecific nest parasitism is frequent in waterfowl and other species with self-feeding young because the costs of parasitism to hosts are minimal and produce little selection for host defenses. I offer several criticisms of this hypothesis and suggest that variation in the rate of conspecific nest parasitism among birds is better understood by focusing on the costs and benefits of parasitism for the parasitic female. I suggest that conspecific nest parasitism is more successful and therefore more frequent in waterfowl species because parasitic eggs and young require no more parental care from the host than it is already providing to its own offspring. In contrast, parasitism is less successful and less frequent in species with parentally fed young because parasitic young require additional care that hosts may not be capable of providing.