Experiments suggest that some hosts that reject parasitic brown-headed cowbird (Molothrus ater) eggs develop egg recognition by imprinting on their own eggs, probably at the first nest naÃ¯ve birds attend. This raises the possibility that parasitism of naÃ¯ve individuals might cause them to mistakenly misimprint on, and accept, cowbird eggs. In an egg-learning experiment designed to cause misimprinting, hosts accepted cowbird eggs when they had laid only 1 egg even though most rejected cowbird eggs when nests had ≥2 host eggs. These findings show that cowbirds can cause misimprinting by parasitizing naÃ¯ve hosts. If done opportunistically, misimprinting should become more likely as cowbird abundance increases. In accord with this expectation, simulated brood parasitism showed that 3 host species that usually reject cowbird eggs were more tolerant of cowbird eggs in the Great Plains, where cowbirds reach maximal historic abundance, than in eastern North America where cowbirds are less common. This relation between parasite abundance and host responses is opposite to that found in cuckoo hosts. In addition, these 3 rejecter species had high rates (10.8--30.4%) of natural cowbird parasitism at an Illinois site where cowbirds are extremely, and historically, abundant. These rejecters accepted most cases of researcher-detected parasitism, even though they rarely accept where cowbirds are less common. Despite the potentially high cost resulting from egg rejection, we demonstrate that parasitism of "rejecters" can be adaptive for cowbirds particularly when host egg recognition involves learning. Copyright 2009, Oxford University Press.