|blue: breeding; green: year-round; ochre: nonbreeding|
The Brown-headed Cowbird (Molothrus ater) is a small brood parasitic icterid of temperate to subtropical North America. They are permanent residents in the southern parts of their range; northern birds migrate to the southern United States and Mexico in winter, returning to their summer habitat around March or April.
The Brown-headed Cowbird is typical for an icterid in general shape but is distinguished by a finch-like head and beak and is smaller than most icterids. The adult male is iridescent black in color with a brown head. The adult female is slightly smaller and is dull grey with a pale throat and very fine streaking on the underparts. The total length is 16–22 cm (6.3–8.7 in) and the average wingspan is 36 cm (14 in). Body mass can range from 30–60 g (1.1–2.1 oz), with females averaging 38.8 g (1.37 oz) against the males' average of 49 g (1.7 oz).
They occur in open or semi-open country and often travel in flocks, sometimes mixed with Red-winged Blackbirds (particularly in spring) and Bobolinks (particularly in fall), as well as Common Grackle or European Starlings. These birds forage on the ground, often following grazing animals such as horses and cows to catch insects stirred up by the larger animals. They mainly eat seeds and insects.
Before European settlement, the Brown-headed Cowbird followed bison herds across the prairies. Their parasitic nesting behaviour complemented this nomadic lifestyle. Their numbers expanded with the clearing of forested areas and the introduction of new grazing animals by settlers across North America. Brown-headed Cowbirds are now commonly seen at suburban birdfeeders.
This bird is a brood parasite: it lays its eggs in the nests of other small passerines (perching birds), particularly those that build cup-like nests. The Brown-headed Cowbird eggs have been documented in nests of at least 220 host species, including hummingbirds and raptors. The young cowbird is fed by the host parents at the expense of their own young. Brown-headed Cowbird females can lay 36 eggs in a season. More than 140 different species of birds are known to have raised young cowbirds.
Host response 
The acceptance of a cowbird egg and rearing of a cowbird can be costly to a host species. In the American Redstart, nests parasitized by cowbirds were found to have a higher rate of predation, likely due in part to the loud begging calls by the cowbird nestling, but also partly explained by the fact that nests likely to be parasitized are also more likely to be predated.
Host birds sometimes notice the cowbird egg, to which different host species react in different ways. Rejection manifests in three forms: nest desertion (e.g., Blue-gray Gnatcatcher), burying of the egg under nest material (e.g., Yellow Warbler), and physical ejection of the egg from the nest (e.g., Brown Thrasher). Brown-headed cowbird nestlings are also sometimes expelled from the nest. The Gray catbird rejects cowbird eggs over 95% of the time; experimentation has suggested that in this species, the cost of accepting an egg is much higher (0.79 catbird fleglings) than the cost of rejecting an egg (0.0022 catbird fledglings).
Nestlings of host species can also alter their behavior in response to the presence of a cowbird nestling. Song sparrow nestlings in parasitized nests alter their vocalizations in frequency and amplitude so that they resemble the cowbird nestling, and these nestlings tend to be fed equally often as nestlings in unparasitized nests.
Parasite response 
It seems that Brown-headed Cowbirds periodically check on their eggs and young after they have deposited them. Removal of the parasitic egg may trigger a retaliatory reaction termed "mafia behavior". According to a study by the Florida Museum of Natural History published in 1983, the cowbird returned to ransack the nests of a range of host species 56% of the time when their egg was removed. In addition, the cowbird also destroyed nests in a type of "farming behavior" to force the hosts to build new ones. The cowbirds then laid their eggs in the new nests 85% of the time.
Human intervention 
Humans sometimes engage in cowbird control programs, with the intention of protecting species negatively impacted by the cowbirds' brood parasitism. A study of nests of Bell's Vireo highlighted a potential limitation of these control programs, demonstrating that removal of cowbirds from a site may create an unintended consequence of increasing cowbird productivity on that site, because with fewer cowbirds, fewer parasitized nests are deserted, resulting in greater nest success for cowbirds.
- BirdLife International (2012). "Molothrus ater". IUCN Red List of Threatened Species. Version 2012.1. International Union for Conservation of Nature. Retrieved 16 July 2012.
- Henninger, W.F. (1906). "A preliminary list of the birds of Seneca County, Ohio". Wilson Bull. 18 (2): 47–60.
- Brown-headed Cowbird, Life History, All About Birds – Cornell Lab of Ornithology. Allaboutbirds.org. Retrieved on 2013-03-09.
- CRC Handbook of Avian Body Masses by John B. Dunning Jr. (Editor). CRC Press (1992), ISBN 978-0849342585.
- Friedman and Kiff, Herbert and Lloyd F. (1985-05-16). "The parasitic cowbirds and their hosts". Proceedings of the Western Foundation of Vertebrate Zoology 2 (4): 225–304.
- Ortega, C.P. (1998) Cowbirds and Other Brood Parasites. University of Arizona Press, Tucson, ISBN 0816515271.
- Jaramillo, Alvaro; Peter Burke (1999). New World Blackbirds: The Iceterids. London: Christopher Helm. p. 382.
- Kozlovic, Knapton, and Barlow, Daniel R., Richard W., and Jon C. (1996). "Unsuitability of the House Finch as a Host of the Brown-Headed Cowbird" (PDF). The Condor 96 (2). Retrieved 2008-07-25.
- Hannon, Susan J.; Wilson, Scott; McCallum, Cindy A. (2009). "Does cowbird parasitism increase predation risk to American redstart nests?". Oikos 118 (7): 1035–1043. doi:10.1111/j.1600-0706.2008.17383.x.
- Sealy, Spencer g. (April 1995). "Burial of cowbird eggs by parasitized yellow warblers: an empirical and experimental study". Animal Behaviour (The Association for the Study of Animal Behaviour) 49 (4): 877–889. doi:10.1006/anbe.1995.0120. Retrieved 2008-07-25.
- Lorenzana, J. C. (2001). "Fitness costs and benefits of cowbird egg ejection by gray catbirds". Behavioral Ecology 12 (3): 325–329. doi:10.1093/beheco/12.3.325.
- Pagnucco, K.; Zanette, L.; Clinchy, M.; Leonard, M. L (2008). "Sheep in wolf's clothing: host nestling vocalizations resemble their cowbird competitor's". Proceedings of the Royal Society B: Biological Sciences 275 (1638): 1061–1065. doi:10.1098/rspb.2007.1706.
- Hoover, Jeffrey P. &. Robinson, Scott K. (2007). "Retaliatory mafia behavior by a parasitic cowbird favors host acceptance of parasitic eggs". PNAS 104 (11): 4479–4483. doi:10.1073/pnas.0609710104.
- Kosciuch, Karl L.; Sandercock, Brett K. (2008). "Cowbird removals unexpectedly increase productivity of a brood parasite and the songbird host". Ecological Applications 18 (2): 537–548. doi:10.1890/07-0984.1. PMID 18488614.
|Wikimedia Commons has media related to: Molothrus ater|
- Brown-headed Cowbird videos, photos & sounds on the Internet Bird Collection
- Brown-headed Cowbird Information at Animal Diversity Web