Cowbirds are birds belonging to the genus Molothrus in the family Icteridae. They are brood parasitic New World birds which are unrelated to the Old World cuckoos, one of which, the Common Cuckoo, is the best-known brood parasitic bird.

This family includes five species of cowbirds that form the natural genus Molothrus. This has been determined by phylogenetic analyses of mitochondrial DNA sequences (Lanyon 1992, Johnson and Lanyon 1999, Lanyon and Omland 1999, in Lowther, 2004).

The genus Molothrus contains:

• Screaming Cowbird, Molothrus rufoaxillaris
• Giant Cowbird, Molothrus oryzivorus (formerly in Scaphidura)
• Bronzed Cowbird, Molothrus aeneus
• Shiny Cowbird, Molothrus bonariensis
• Brown-headed Cowbird, Molothrus ater

The non-brood parasitic Bay-winged Cowbird was formerly placed in this genus; it is now Agelaioides badius.

These birds feed on insects, including the large numbers that may be stirred up by cattle. In order for the birds to remain mobile and stay with the herd, they have adapted by laying their eggs in other birds' nests. The cowbird will watch for when its host lays eggs, and when the nest is left unattended, the female will come in and lay its own eggs. The female cowbird may continue to observe the nest after laying her eggs. If the cowbird egg is removed, the female cowbird may destroy the host's eggs^[1].

The Brown-headed Cowbird has over 220 hosts^[2]. The other cowbird species have fewer known hosts, but all the species are generalists when it comes to choosing a host. This means that the eggs may look different from the hosts' eggs. The cowbird chicks grow quickly, and may consume most of the food the host brings. Starvation will often kill the host's chicks. In some species the cowbird chick will use its large size to push the other chicks out of the nest. The Giant Cowbird does not appear to harm its host oropendola or cacique chicks.

Brown-headed Cowbird, adult male

Brown-headed Cowbird, courtship behavior

References

1. ^ Jeffrey P. Hoover; Scott K. Robinson (13 March 2007). "Retaliatory mafia behavior by a parasitic cowbird favors host acceptance of parasitic eggs". Proceedings of the National Academy of Sciences. http://www.pnas.org/content/104/11/4479.abstract. Retrieved 26 August 2009. 
2. ^ Friedmann, Herbert; LLoyd E. Kiff and Stephen I. Rothstein (1977). "A further Contribution to knowledge of the Host Relations of the Parasitic Cowbirds.". The Smithsonian. Smithsonian Institution Press. http://si-pddr.si.edu/dspace/handle/10088/5409. Retrieved 26 August 2009. 

• Jaramillo and Burke, New World Blackbirds ISBN 0-7136-4333-1