Cuckoos, cowbirds and the persistence of broad parasitism

Brood parasites provide a particularly good opportunity for the study of ho st-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 rela tionships with their hosts, yet share the attribute of having been particul arly well studied by biologists. Recent work an the cuckoo and the cowbird has resulted ire new answers to the question begged by all brood parasites: why do host species raise parasitic young?