Cuckoos, cowbirds and the persistence of brood parasitism

Brood parasites provide a particularly good opportunity for the study of host–parasite evolution because they directly affect the reproductive success of their hosts. Two parasitic species, the common cuckoo (Cuculus canorus) and the brown-headed cowbird (Molothrus ater), differ widely in their relationships with their hosts, yet share the attribute of having been particularly well studied by biologists. Recent work on the cuckoo and the cowbird has resulted in new answers to the question begged by all brood parasites: why do host species raise parasitic young?