Parasitism is an interaction in which one organism (the parasite) co-opts the resources of another organism (the host). By definition, the host is hurt by the association while the parasite benefits. Parasitism can be physical, like the parasitic worms found in the internal organs of animals, or social, like the brood or nest parasitism found in some birds. In these species, resident birds are tricked into incubating the eggs (and raising the chicks) of interlopers; one good example in the bird world is the cowbird.