Molothrus ater (brown-headed cowbird)
- Summary of Invasiveness
- Taxonomic Tree
- Distribution Table
- Habitat List
- Biology and Ecology
- Means of Movement and Dispersal
- Impact Summary
- Threatened Species
- Risk and Impact Factors
- Similarities to Other Species/Conditions
- Prevention and Control
- Links to Websites
- Principal Source
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Molothrus ater (Boddaert, 1783)
Preferred Common Name
- brown-headed cowbird
International Common Names
- English: buffalo bird; cowbird
- MOLOAT (Molothrus ater)
Summary of InvasivenessTop of page
M. ater, the brown-headed cowbird, is a small bird of the family Icteridae. Like other cowbirds (genus Molothrus) it is a brood parasite, laying its eggs in the nests of other species - this reduces the reproductive success of the birds that it parasitizes, and parasitism by the brown-headed cowbird has been seen as an important threat to certain bird species. M. ater can be found in a variety of habitats, including 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. M. ater has undergone a rapid range expansion associated with habitat alterations, such as forest clearing, domestic cattle grazing, urbanization and conversion of forested habitats to agricultural land.
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Metazoa
- Phylum: Chordata
- Subphylum: Vertebrata
- Class: Aves
- Order: Passeriformes
- Family: Icteridae
- Genus: Molothrus
- Species: Molothrus ater
DescriptionTop of page
M. ater is a small bird 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.
DistributionTop of page
Native range: Historically, M. ater was largely confined to North America’s mid-continental prairies.
Known introduced range: M. ater underwent a rapid range expansion and invaded the Great Lakes region 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 across the entire continental USA to central Mexico. They winter throughout this range, and also in southern Mexico and the tip of Florida. The expansion of M. ater from its native range is associated with habitat alterations such as the conversion of forested lands to farms and pastures.
Distribution TableTop of page
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|Bermuda||Present, few occurrences||Avibase, 2012|
|Canada||Present||Present based on regional distribution.|
|-British Columbia||Present||ISSG, 2011|
|-New Brunswick||Present||ISSG, 2011|
|-Newfoundland and Labrador||Present||ISSG, 2011|
|-Northwest Territories||Present||ISSG, 2011|
|-Nova Scotia||Present||ISSG, 2011|
|-Prince Edward Island||Present||ISSG, 2011|
|-Yukon Territory||Present||ISSG, 2011|
|Saint Pierre and Miquelon||Present||Avibase, 2012|
|USA||Present||Present based on regional distribution.|
|-District of Columbia||Present||ISSG, 2011|
|-New Hampshire||Present||ISSG, 2011|
|-New Jersey||Present||ISSG, 2011|
|-New Mexico||Present||ISSG, 2011|
|-New York||Present||ISSG, 2011|
|-North Carolina||Present||ISSG, 2011|
|-North Dakota||Present||ISSG, 2011|
|-Rhode Island||Present||ISSG, 2011|
|-South Carolina||Present||ISSG, 2011|
|-South Dakota||Present||ISSG, 2011|
|-West Virginia||Present||ISSG, 2011|
Central America and Caribbean
|Bahamas||Present, few occurrences||Avibase, 2012|
|Belize||Present, few occurrences||Avibase, 2012||Orange Walk District (Avibase, 2012)|
|Cuba||Present, few occurrences||Avibase, 2012|
|Turks and Caicos Islands||Present, few occurrences||Avibase, 2012|
HabitatTop of page
M. ater prefers 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 include open coniferous and deciduous woodlands, forest edges, brushy thickets, agricultural land and suburban areas (Rothstein 1994, in Sullivan, 1995).
Habitat ListTop of page
|Terrestrial – Managed||Cultivated / agricultural land||Present, no further details|
|Managed forests, plantations and orchards||Present, no further details|
|Managed grasslands (grazing systems)||Present, no further details|
|Disturbed areas||Present, no further details|
|Urban / peri-urban areas||Present, no further details|
|Terrestrial ‑ Natural / Semi-natural||Natural forests||Present, no further details|
|Scrub / shrublands||Present, no further details|
Biology and EcologyTop of page
M. ater often feeds 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
M. ater begins breeding in April breeding peaks in May, starts declining in June, and occurs sporadically through July (GMNH 2000). The brown-headed cowbird is an obligate brood parasite 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 after a further 10-13 days, with the host adults successfully raising at least one hatchling.
M. ater is a generalist brood parasite, and has been recorded to lay eggs in the nests of 220 species, although successful parasitism has only been recorded in 144 of these species - either because the host rejects the cowbird egg or because the host is an inadequate foster parent. Because of this, the vast majority of cowbird eggs deposited (~97%) do not result in a cowbird adult. In addition to laying its own egg, if there is more than one host egg in the nest already, the cowbird also removes or occasionally eats a host egg (Stanford University, 2012). Cowbird eggs usually have shorter incubation periods and chicks are larger than host chicks. These advantages enable cowbird chicks to outcompete host chicks for resources, which can result in the loss of the entire host brood (US Fish and Wildlife Service, 2010b).
Means of Movement and DispersalTop of page
Introduction pathways to new locations
Agriculture:M. ater has experienced rapid range expansion associated with habitat alterations including 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).
Impact SummaryTop of page
ImpactTop of page
Original text compiled by IUCN SSC Invasive Species Specialist Group (ISSG)
M. ater, the brown-headed cowbird, is a brood parasite. It operates by using other bird species as hosts to incubate its eggs and raise its chicks. The brown-headed cowbird reduces the reproductive success of the species it parasitizes – firstly, because the female cowbird removes a host egg from the nest on the day, or day before it lays its own egg (Stanford, 2012). Also, if the parent of the host species fosters the cowbird egg, this increases energetic demands, negatively affecting provision of its own hatchlings (Pappas, 2010); alternatively, the parent of the host species, instead of fostering the cowbird egg, may abandon a parasitized nest, which also reduces reproductive success (Stanford University, 2012).
The brown-headed cowbird is known to have parasitized over 220 host species, including the black-capped vireo (Vireo atricapilla), the wood thrush (Hylocichla mustelinus), the blue-winged teal (Anas discors) and the red-headed woodpecker (Melanerpes erythrocephalus). 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 bird species, most of them being songbirds. In recent decades, many people including land managers and conservationists have said that parasitism by brown-headed cowbirds is a major threat to songbird populations in North America and is responsible for range-wide population declines in a number of songbird species. Although some host species are able to re-nest after being parasitized, (hence, reducing the damage to reproductive productivity caused by the parasitism), other species notably those with shorter breeding seasons are unable to re-nest. Such species tend to suffer most heavily from cowbird parasitism. Some authors have found that cowbird parasitism is most frequent at the edge of a patch of habitat compared with the interior of a habitat (Stumpf et al., 2011). This is most well established in the case of forest edge habitat - cowbirds tend to avoid large unbroken swathes of forest interior but frequently parasitize nests at the edge of forests (Lebbin et al., 2010).
The impacts of cowbirds on their host species are disputed. In a review on the topic, the National Audubon Society argued that conservationists and the public tend to overestimate the significance of parasitism as a major cause of declining songbird populations (Meuhter, 2003). Rothstein (2004) also discusses the impact of cowbird parasitism with scepticism - see the review article: 'Brown-headed cowbirds: villain or scapegoat?'. Nevertheless, Stumpf et al. (2012) notes that in the case of small populations of some endangered birds, nest parasitism by M. ater has been identified as an important factor limiting reproductive success, albeit in conjunction with other factors such as nest predation and a decline in habitat (see examples below). Because brown-headed cowbirds are generalist parasites, the extinction of a single rare host species has little or no effect on their ability to find suitable hosts, so there is no strong feedback effect on the cowbird population, since they can find other species to parasitize (Griffith and Griffith, 2000).
The Kirtland Warbler (Dendroica kirtlandii) is a neo-tropical migratory songbird that nests in pine forests in Michigan. It was found that 69% of nests examined over a 5 year period (1966-1971) had been parasitized with M. ater eggs. Annual trapping of brown-headed cowbirds formed part of the conservation strategy, and it is now believed that only around 3% of nests are parasitized (IUCN, 2012; US Fish and Wildlife Service, 2010b). In the early 1980’s the endangered Bell’s vireo (Vireo bellii) was also recorded to have high levels of brown-headed cowbird parasitism; due to the bird’s small size, parasitism often leads to complete reproductive failure. Although habitat loss had contributed to its decline, parasitism is also an important factor; hence, since cowbird trapping began, V. bellii populations have stabilized and then grown and expanded even though other factors are not believed to have changed (see Griffith and Griffith, 2000).
Threatened SpeciesTop of page
|Threatened Species||Conservation Status||Where Threatened||Mechanism||References||Notes|
|Dendroica kirtlandii||NT (IUCN red list: Near threatened) NT (IUCN red list: Near threatened)||Florida; Michigan; South Carolina; Wisconsin||Parasitism (incl. parasitoid)||US Fish and Wildlife Service, 1985|
|Polioptila californica californica (coastal California gnatcatcher)||USA ESA listing as threatened species USA ESA listing as threatened species||California||Ecosystem change / habitat alteration||US Fish and Wildlife Service, 2010a|
Risk and Impact FactorsTop of page Impact outcomes
- Ecosystem change/ habitat alteration
- Threat to/ loss of native species
- Parasitism (incl. parasitoid)
Similarities to Other Species/ConditionsTop of page
According to GMNH (2000), the adult male Molothrus aeneus lacks the brown head of M. ater and has red eyes in contrast to the brown-headed cowbird's black eyes; the female M. aeneus also has reddish eyes. The female M. aeneus is variable in colour but darker than the female M. ater. Like the brown-headed cowbird, the range of M. aeneus has expanded in the last few decades.
Originally from the Greater Antilles, but now spreading through the Lesser Antilles, and established in Florida and Georgia, with individuals appearing west to Texas.
Prevention and ControlTop of page
Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.
Original text compiled by the IUCN SSC Invasive Species Specialist Group (ISSG)
Physical: M. ater 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).
Trapping programmes have been used with success to control the threat of brown-headed cowbirds to the Kirtland’s warbler (Dendroica kirtlandii) and Bell’s vireo (Vireo bellii). In both cases, decoy traps were used; in other words, the traps contained a live cowbird. Being a gregarious species, the ‘decoy bird’ helps attract other brown-headed cowbirds to the trap (see US Fish and Wildlife Service, 2010b and Griffith and Griffith, 2000 for further details).
Habitat management approaches: M. ater is known to parasitize nests in forest edge habitat but does not usually inhabit the forest interior. Therefore, managing the landscape in a way that reduces forest edges could reduce cowbird parasitism. For example, preventing habitat fragmentation and maintaining large patches of forest with high edge:area ratios would achieve the goal of reducing forest edges (Lebbin et al., 2010).
Amur honeysuckle (Lonicera maackii) is a non-native species in North America and is thought to increase the risk of brood parasitism by brown-headed cowbirds on Acadian flycatchers (Empidonax virescens). Therefore, removal of invasive Amur honeysuckle could contribute towards a cowbird parasitism control strategy in certain cases (Rodewald, 2011).
BibliographyTop of page
Smith JNM, Cook TL, Rothstein SI, Robinson SK, Sealy SG, eds., 2000. Ecology and management of cowbirds and their hosts. Austin, USA: University of Texas Press, 400 pp.
Lowther PE, 2011. Lists of victims and hosts of the parasitic cowbirds (Molothrus).
References from GISD
eNature.com, 2007. Bringing nature to life: Brown-headed cowbird. Molothrus ater http://www.enature.com/fieldguides/detail.asp?allSpecies=y&searchText=brown%20headed%20cowbird&curGroupID=1&lgfromWhere=&curPageNum=1
Meuhter, V. 2003. Cowbirds and Conservation. National Audubon Society. http://www.audubon.org/bird/research/
Roof, J. 1997. Molothrus ater: brown-headed cowbird. University of Michigan. http://animaldiversity.ummz.umich.edu/accounts/molothrus/m._ater.html
Sullivan, J. 1995. Molothrus ater. Fire Effects Information System. http://www.fs.fed.us/database/feis/animals/bird/moat/all.html
The Georgia Museum of Natural History (GMNH). 2000. Molothrus ater. Georgia Department of Natural Resources.
ReferencesTop of page
Avibase, 2012. Avibase - the world bird database. http://avibase.bsc-eoc.org
Griffith JT, Griffith JC, 2000. Cowbird control and the endangered least bell's vireo: a management success story. 342-356. In: Ecology and management of cowbirds and their hosts [ed. by Smith, J. N. M. \Cook, T. L. \Rothstein, S. I. \Robinson, S. K. \Sealy, S. G.]. University of Texas Press, Austin, USA.
IUCN, 2012. Dendroica kirtlandii (Kirtland's Warbler). http://www.iucnredlist.org/details/106009114/0.
Lebbin DJ, Parr MJ, Fenwick GH, 2010. The American Bird Conservancy Guide to Bird Conservation., Chicago: University of Chicago Press, 456 pp.
NatureServe, 2012. InfoNatura. Animals and Ecosystems of Latin America. Molothrus ater - Brown-headed Cowbird. http://www.natureserve.org/infonatura/.
Pappas S, Benson TJ, Bednarz JC, 2010. Effects of Brown-Headed Cowbird Parasitism on Provisioning Rates of Swainson's Warblers. The Wilson Journal of Ornithology, 122(1): 75-81.
Rodewald D, 2011. Spreading messages about invasives. Diversity and Distributions, 18(1): 97-99.
Rothstein SI, 2004. Brown-headed cowbird: Villain or scapegoat? Birding. http://bcrc.bio.umass.edu/courses/spring2006/biol/biol544/cowbird.pdf.
Smith JNM, Cook TL, Rothstein SI, Robinson SK, Sealy SG, 2000. Ecology and management of cowbirds and their hosts. Austin, USA: University of Texas Press, 400 pp.
Stanford University, 2012. Stanford birds. Cowbirds. http://www.stanford.edu/group/stanfordbirds/text/essays/Cowbirds.html.
Stumpf KJ, Theimer TC, McLeod MA, Koronkiewicz TJ, 2012. Distance from riparian edge reduces brood parasitism of southwestern willow flycatchers, whereas parasitism increases nest predation risk. Journal of Wildlife Management, 76(2):269-277. http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1937-2817
US Fish and Wildlife Service, 2010. In: Coastal California gnatcatcher (Polioptila californica californica). 5-Year Review: Summary and Evaluation. US Fish and Wildlife Service, 51 pp.. http://www.fws.gov/ecos/ajax/docs/five_year_review/doc3571.pdf
US Fish and Wildlife Service, 2010. USA: US Fish and Wildlife Service, Brown-headed cowbird control in Kirtland's warbler nesting areas, Northern Lower Michingan. http://www.dodpif.org/kiwa/kw-reports/Cowbird%20Reports%201973-2010/2010%20Cowbird%20Trapping%20Report.pdf.
ContributorsTop of page
- Reviewed by: Dr. Keith Arnold. Professor. Department of Wildlife and Fisheries Sciences, Texas A and M University, USA
- Last Modified: Monday, January 24, 2005
Distribution MapsTop of page
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