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Molothrus ater

Molothrus ater

Cladus: Eukaryota
Supergroup: Opisthokonta
Regnum: Animalia
Subregnum: Eumetazoa
Cladus: Bilateria
Cladus: Nephrozoa
Cladus: Deuterostomia
Phylum: Chordata
Subphylum: Vertebrata
Infraphylum: Gnathostomata
Superclassis: Tetrapoda
Classis: Aves
Subclassis: Carinatae
Infraclassis: Neornithes
Parvclassis: Neognathae
Ordo: Passeriformes
Subordo: Passeri
Parvordo: Passerida
Superfamilia: Passeroidea
Familia: Icteridae
Genus: Molothrus
Species: Molothrus ater
Subspecies: M. a. artemisiae - M. a. ater - M. a. obscurus


Molothrus ater (Boddaert, 1783)


Table des Planches Enluminéez d'Histoire Naturelle de M. D'Aubenton p.37

Vernacular names
English: Brown-headed Cowbird
Nederlands: Bruinkop-koevogel
Polski: Starzyk brunatnogłowy

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.[1]

They resemble New World orioles in general shape but have a finch-like head and beak. The adult male is mainly iridescent black with a brown head while the adult female is grey with a pale throat and fine streaking on the underparts.


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.[1] 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.[2][3] 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.[4]

Unlike the Common Cuckoo, it has no gentes whose eggs imitate those of a particular host.

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),[5] and physical ejection of the egg from the nest (e.g., Brown Thrasher).[3] Brown-headed cowbird nestlings are also sometimes expelled from the nest.

Some species, such as the House Finch feed their young a vegetarian diet. This is unsuitable for young Brown-headed Cowbirds, meaning almost none survive to fledge.[6]

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.[7]


^ a b Henninger (1906)
^ 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.
^ a b Ortega (1998)
^ Jaramillo, Alvaro; Peter Burke (1999). New World Blackbirds: The Iceterids. London: Christopher Helm. pp. 382.
^ 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.
^ 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.
^ Hoover & Robinson (2007)


BirdLife International (2004). Molothrus ater. 2006. IUCN Red List of Threatened Species. IUCN 2006. www.iucnredlist.org. Retrieved on 5 May 2006. Database entry includes justification for why this species is of least concern
Henninger, W.F. (1906): A preliminary list of the birds of Seneca County, Ohio. Wilson Bull. 18(2): 47-60. DjVu fulltext PDF fulltext
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 PDF fulltext
Ortega, C.P. (1998) Cowbirds and Other Brood Parasites. University of Arizona Press, Tucson.

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