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   Molothrus ater (bird)   
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      Brown-headed cowbird (Photo: Dan Sudia) - Click for full size   Molothrus ater (Photo: Giff Beaton) - Click for full size
    Taxonomic name: Molothrus ater (Boddaert, 1783)
    Common names: brown-headed cowbird (English), buffalo bird (English), cowbird (English)
    Organism type: bird
    Molothrus ater is a small blackbird that can be found in almost every habitat, open woodlands, fields and marginal habitats in between. It is commonly associated with agriculture (cattle pastures, feed lots) and is migratory, spending time year-round in the southern United States, but occurring only during the breeding season in the northern and mountainous regions of the United States. Molothrus ater has undergone a rapid range expansion with habitat alterations, due to forest clearing, domestic cattle grazing, urbanisation and conversion of forested habitats to agricultural land.
    Molothrus ater (brown-headed cowbird) is a small blackbird with a short, conical bill and long, pointed wings (Roof 1997). Males appear black with a unique brown head and neck. Females are either dullish grey or brown throughout. The bill is a dull grey and the eyes are black.
    Similar Species
    Molothrus aeneus, Molothrus bonariensis

    Occurs in:
    agricultural areas, natural forests, planted forests, range/grasslands, ruderal/disturbed, scrub/shrublands, urban areas
    Habitat description
    Molothrus ater (brown-headed cowbird) prefer open habitats of low or scattered trees interspersed with grasslands; they usually avoid unbroken forest. Brown-headed cowbirds prefer, and may require, areas of short grass or bare ground for foraging. They are commonly associated with cattle pastures and feedlots (GMNH, 2000). Other habitats are open coniferous and deciduous woodlands, forest edges, brushy thickets, agricultural land and suburban areas (Rothstein 1994, in Sullivan, 1995).
    General impacts
    Molothrus ater; the brown-headed cowbird, is a brood parasite. It operates by using other bird species as hosts to incubate their eggs and raise its chicks. It is known to have parasatised over 220 host species of birds, including the black-capped vireo, the wood thrush,the blue-winged teal and the red-headed woodpecker. Although some bird species reject brown-headed cowbird eggs, and thus cannot become hosts for the parasite, cowbird chicks are successfully reared by over 150 species, most of them being songbirds.In recent decades, many people including land managers, conservationists, and citizens have said that parasatism by brown-headed cowbirds is a major threat songbird populations in North America and is responsible for range-wide population declines in a number of songbird species. Although some species are able to renest, and therefore not lose much reproductive success because of the cowbird, other species which have shorter breeding seasons are not, and are therefore in more danger from its parasatism.
    Some of these impacts are controversial however, and in a recent review of the extent to which cowbrids harm bird populations in North America, Audubon Science argued that conservationists and the public tend to overestimate the significance of parasitism as a major cause of declining songbird populations. (Meuhter, 2003)
    Geographical range
    Native range: Historically, Molothrus ater was largely confined to North America’s mid-continental prairies.
    Known introduced range: Molothrus ater underwent a rapid range expansion and invaded the Great Lakes states and the northeastern U.S. during the nineteenth century (Robbins et al. 1986, in Sullivan, 1995). It has also spread westward to California (Rothstein 1994, in Sullivan, 1995). According to Roof (1997), M. ater breeds from southeast Alaska, through lower Canada, and through the entire continental U.S. to central Mexico. They winter throughout this range, and also in southern Mexico and the tip of Florida.
    Introduction pathways to new locations
    Agriculture: Molothrus ater (brown-headed cowbird) experienced rapid range expansion with habitat alterations due to forest clearing, domestic cattle grazing, urbanization, and conversion of forested habitats to agricultural land (Roof 1997).

    Local dispersal methods
    Agriculture (local): Land cleared for agriculture has represented additional, preferred feeding grounds (GMNH, 2000).
    Management information
    Physical: Molothrus ater (brown-headed cowbird) is managed through lethal control: trapping and killing of adults and removal of eggs from host nests. Trapping is seen as the most efficient tool for removing large numbers (Muehter 2003).
    Molothrus ater (brown-headed cowbird) often feed on the ground, away from vegetation (Roof 1997). Their main food items are seeds and arthropods. They sometimes hawk, looking for slow flying insects. In a quantitative analysis of the brown-headed cowbird's diet, it was found that nearly 75% was 'weed' seed, with most of the remaining 25% made up of grasshoppers and beetles
    Lifecycle stages
    Molothrus ater (brown-headed cowbird) breeding begins in April, peaks in May, starts declining in June, and occurs sporadically through July (GMNH 2000). The brown-headed cowbird is parasitic and therefore builds no nest. The female can lay approximately 40 eggs in one breeding season, usually 1-2 in each host's nest. The eggs usually hatch after 10-13 days, and the young fledge in 10-13 days, with the host adults successfully raising at least one hatchling.
    Reviewed by: Dr. Keith Arnold. Professor. Department of Wildlife and Fisheries Sciences, Texas A and M University, USA
    Compiled by: National Biological Information Infrastructure (NBII) & IUCN/SSC Invasive Species Specialist Group (ISSG)
    Last Modified: Monday, 24 January 2005

ISSG Landcare Research NBII IUCN University of Auckland