Brown-headed Cowbird
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 Adult male
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| Molothrus ater (Boddaert, 1783) |
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 about March/April.[1]
They resemble New World orioles in general shape but have a finch-like head and beak. Adults have a short finch-like bill and dark eyes. The adult male is mainly iridescent black with a brown head. The adult female is grey with a pale throat and fine streaking on the underparts.
[edit] Ecology
Habitat 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.
Reproduction 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 over 200 host species (Ortega 1998) 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. Over 140 different species of birds are known to have raised young cowbirds[citation needed].
Unlike the Common Cuckoo, it has no gentes whose eggs imitate those of a particular host.
[edit] Host Parasite Interactions
Egg Rejection Host parents may sometimes easily 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)(Ortega 1998). Brown-headed cowbird nestlings are sometimes expelled from the nest.
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 2007, the cowbird returned to ransack the nests of a range of host species in 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.[2]
[edit] Footnotes
[edit] References
- BirdLife International (2004). Molothrus ater. 2006 IUCN Red List of Threatened Species. IUCN 2006. 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.
