This study is the first systematic attempt to investigate experimentally the presence or absence of antiparasite adaptations in a large number of actual and potential host species of a parasitic bird. Experiments were conducted by adding artificial Brown-headed Cowbird eggs to 640 nests of 43 species of North American passerine and other small birds. Within each species studied, all individuals tended to either accept or reject cowbird eggs, and the species are easily separable into "accepters" and "rejecters." Of the species with sufficient data, 23 are accepters and 7 are rejecters. Previously, only two North American species were known to reject cowbird eggs frequently. In the absence of experimental data it is not surprising that the true status of the other rejecters has gone undetected since there may be much unseen natural cowbird parasitism with rejecters removing cowbird eggs before observers determine that parasitism has occurred. The results of this study indicate that most North American birds lack host defenses, and support the belief that the host-parasite system of the Brown-headed Cowbird is not a highly evolved interaction.