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Cowbirds are belonging to the genus Molothrus in the family Icteridae. They are of New World origin, but some species not native to North America are invasive there, and are obligate , laying their eggs in the nests of other species.
The genus was introduced by English naturalist William Swainson in 1832 with the brown-headed cowbird ( Molothrus ater) as the type species. The title page bears the year 1831 but the volume did not appear until 1832. The genus name combines the Ancient Greek , meaning "struggle" or "battle", with , meaning "to sire" or "to impregnate". The English name "cowbird", first recorded in 1839, refers to this species often being seen near cattle.
One Extinction species, Molothrus resinosus, is known from fossil remains recovered from the Talara Tar Seeps of northwestern Peru, and likely went extinct during the late Quaternary. It may have been a close associate of Pleistocene megafauna communities, and may have gone extinct following their collapse in populations. The convex-billed cowbird ( Pandanaris convexa) is another extinct species that likely co-evolved with the megafauna, though it is placed in its own genus.
The nonparasitic were formerly placed in this genus; they are now classified as Agelaioides.
Cowbirds reproduce by laying their eggs in other birds' nests. Female cowbirds observe a potential host bird laying its eggs, and when the nest is left momentarily unattended, the cowbird lays its own egg in it. The female cowbird may continue to observe this nest after laying eggs. Some bird species have evolved the ability to detect such parasitic eggs, and may reject them by pushing them out of their nests, but female cowbirds have been observed to attack and destroy the remaining eggs of such birds in retaliation, as suggested by the Mafia hypothesis.
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