For many species of birds, building a nest and raising a chick is a long, energy-depleting job. Several species have found a way around this parental occupation by laying their eggs in the nest of other birds. These species are known as brood parasites.
Parasitic birds all exhibit the same practice, called cuckoldry, in which they deposit their eggs in the nest of another bird species, and leave the chick to be raised by its surrogate parent. Parasitic bird species include Black-headed ducks and cowbirds in the Americas, old-world cuckoos in Europe and Australia, and whydahs and honeyguides in Africa.
These birds possess several unique adaptations that enable them to function as a brood parasite. Most birds are specialists that will only parasitize the nests of a single species or a group of related species, but several are generalists. For example, the cowbird has been documented parasitizing the nests of 221 other bird species. Cowbirds can also lay two eggs in its host’s nest, while specialist brood parasites generally lay a single egg.
The eggs of parasitic species often mimic the appearance of the host species’ eggs. Cowbirds have even evolved egg colorations that mimic the eggs of several hosts. When the female lays its egg in the nest of another bird, she usually disposes of any of the host’s eggs in the nest. This action prevents the host from realizing is has been parasitized and reduces competition for its own chick. If the host lays another egg, however, the brood parasite chick can still maintain an advantage.
Brood parasite chicks often have a shorter incubation period and grow more rapidly than their host chicks. Using this strategy, a parasitic chick will hatch before a host chick and be able to outcompete the host chicks, or even kill its nest mates or kick unhatched eggs out of the nest. Once the chick has dispatched its rivals, it demands the full attention of its host parent, and grows quickly. Poignant photographs often depict a Brown-headed cowbird chick, comically large in the nest of a small songbird, gaping for food delivered by a host species so petite that it’s surprising that the chick doesn’t consume its benefactor.
Another incredible adaptation exhibited by brood parasites, particularly cowbirds, is their innate and foolproof ability to know that they are cowbirds, and not a member of its host parent’s species. A cowbird has never been documented attempting to court another species or singing a song other than its own. The mechanism for this innate knowledge remains unclear, but scientists believe that cowbirds development of species recognition and song is not learned, as in other species.
Species that are repeated target hosts for parasitic birds have developed strategies to avoid nest parasitism. Some species design their nests to be difficult to parasitize, or begin incubation early so that adults are sitting on the nest when parasitic birds visit early in the morning. Some species can even recognize a parasite’s eggs and remove it from the nest.
