The genus was introduced by English naturalist William Swainson in 1832 with the brown-headed cowbird (Molothrus ater) as the type species.

It may have been a close associate of Pleistocene megafauna communities, and may have gone extinct following their collapse in populations.

[7] 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.

Some species follow ungulates to catch insects stirred up by the larger animals' grazing.

Some bird species have evolved the ability to detect such parasitic eggs, and may reject them by pushing them out of their nests, but the female cowbird has been observed to attack and destroy the remaining eggs of such birds as a consequence, dissuading further removals.