Brown-headed Cowbird

Distribution and Abundance

• BBS Map | CBC Map
• Breeding range entire United States, southern Canada and western Canada.
• Before European settlement, range limited to open grasslands of central North America; spreading east in early 1800s as forests were cleared, creating open habitat for expansion (Mayfield 1965). Similar expansion west in the 1900s (Rothstein et al. 1980, Laymon 1987).
• Short-distance migrant within North America; seasonal migration between breeding and wintering distributions. Northeast U.S. birds travel about 800-850km between breeding season banding and winter recovery (Dolbeer 1982).
• Winter range in eastern, southern central, and west coast United States; into Mexico.

Habitat

• Prefers habitats with low or scattered trees among grassland vegetation including woodland edges, brushy thickets, prairies, fields, pastures, orchards, and residential areas. Wood-field ecotones preferred over either extensive woods or extensive field or prairie areas (Gates and Gysel 1978, Brittingham and Temple 1983, Johnson and Temple 1990).
• In Wisconsin, of 1375 observations from 1995-2000 where habitat type was reported, 25% were in upland hardwood shrub (particularly hedgerows) or upland mixed shrub, 23% were in urban areas, 19.5% were in upland hardwood forests (particularly oak) or upland mixed forests, and 14% in upland open areas (WSO 2002). In Michigan, one-third of all observations from 1983-1988 were in residential-roadside-hedgerow habitats (Brewer et al. 1991).
• Forages on ground, rarely in vegetation; frequently in association with ungulates which disturb insect prey.
• Migratory roost sites generally early successional forest, 18-34 years-old, often high density stands of red maple and sweetgum with compact enclosed canopies (Lyon and Caccamise 1981).
• In winter, generally found at the margins in areas with deciduous trees (Robertson et al. 1978).

Behavior

• Diet predominately seeds and arthropods.
• Populations vary in primary type of mating system (promiscuous, polygynous, polyandrous or monogamous), possibly in relation to population sex ratios (Teather and Robertson 1986)  or population density (Yokel 1989). Pairs may continue over successive years (Dufty 1982). 
• Mated males guard females from courtship behavior of other males (Yokel 1986, 1989) and respond aggressively to calls of other males (Rothstein et al. 1988); defense but not mutual exclusion of territories (Teather and Robertson 1985).
• Females respond to calls of other females with aggressive approaches (Darley 1982, Yokel 1989), but laying ranges overlap (Fleischer 1985); thus more than one female may lay in the same host nest. Feeding areas overlap laying ranges; more so in eastern studies (Darley 1982, 1983; Dufty 1982).
• Birds can travel several kilometers between foraging sites, areas with little or short vegetation (such as lawns), and nests that are parasitized (references).
• No indication of interspecific territoriality.
• Gregarious, often found in mixed-species flocks of blackbirds.

Parasitism and Predation

• Over 220 host species have been reported as parasitized; 144 species have reared cowbird young (Friedmann et al. 1977, Friedmann and Kiff 1985). 
• Many bird species respond aggressively to Brown-headed Cowbirds, an indication that hosts can identify cowbirds as a threat to their own reproductive success (Robertson and Norman 1976, 1977; Folkers and Lowther 1985, Smith et al. 1984, Burgham and Picman 1989).
• In Wisconsin, of confirmed breeding observations from 1995-2000, the top hosts were Chipping Sparrow and Song Sparrow (WSO 2002). In Michigan, of confirmed breeding observations from 1983-1988, the top host was Kirtland's Warbler, then Field Sparrow and Chipping Sparrow (Brewer et al. 1991). 
• Parasitism rates tend to be relatively low in largely intact forested landscapes of northern Michigan, Wisconsin and Minnesota (references).
• Egg and nestling loss for the cowbird reflects nest predation on the host species. Rejection of cowbird eggs occurs by either ejecting cowbird eggs from the nest, burying cowbird eggs under a new nest lining, or deserting the nest. Certain species (Eastern Kingbird, Blue Jay, American Robin, Gray Catbird, Brown Thrasher, Cedar Waxwing, Northern Oriole) almost always reject cowbird eggs. Most other hosts reject less frequently, less than 20% of the time (Rothstein 1975). Species may include both rejector and acceptor populations depending on overlap in distribution with cowbirds (Briskie et al. 1992).
• Specific predators of fledglings include black racers, black rat snake, and Blue Jays (Woodward and Woodward 1979).

Conservation and Management

• Forest interior species, formerly isolated from cowbird parasitism by habitat, became exposed to parasitism as forest fragmentation increased edge and provided access to cowbirds (Brittingham and Temple 1983).
• Brood parasitism imposes additional mortality on host populations, but stable equilibria are achieved if parasitism rates remain below some critical level (May and Robinson 1985). 
• Present concerns involve species with small populations (e.g., Kirtland's Warbler, Black-capped Vireo, Least Bell's Vireo). 
• Management plans involve trapping and removing cowbirds from host breeding areas (Walkinshaw 1983, Grzybowski et al. 1986).
• BBS trend results from 1966-2000 (Sauer et al. 2001) in the Northern Spruce-Hardwoods region indicate a significant population decline of the Brown-headed Cowbird in this region  (-5.6, p=0.00 Trend Graph S28), as is the case in the Great Lakes Transition region (-2.4, p=0.00 Trend Graph S20). Survey-wide (US and Canada), this species has shown a slight but significant decline (-1.0, p=0.00 Trend Graph SUR).

For more information about the management of the Brown-headed Cowbird, please see the Species Management Abstract, from the Conserve Online public library, maintained by The Nature Conservancy.

References

• Brewer, R., G.A. McPeek and R.J. Adams, Jr. 1991. The atlas of breeding birds of Michigan. Michigan State University Press, East Lansing.
• Briskie, J.V., S.G. Sealy and K.A. Hobson. 1992. Behavioral defenses against avian brood parasitism in sympatric and allopatric host populations. Evolution 46:334-340.
• Brittingham, M.C. and S.A. Temple. 1983. Have cowbirds caused forest songbirds to decline? BioScience 33:31-35.
• Burgham, M.C.J. and J. Picman. 1989. Effect of Brown-headed Cowbird on the evolution of Yellow Warbler anti-parasite strategies. Animal Behavior 38:298-308.
• Darley, J.A. 1982. Territoriality and mating behavior of the male Brown-headed Cowbird. Condor 84:15-21.
• Darley, J.A. 1983. Territorial behaviour of the female Brown-headed Cowbird. Can. J. Zool. 61:65-69.
• Dolbeer, R.A. 1982. Migration patterns for age and sex classes of blackbirds and starlings. J. Field Ornithol. 53:28-46.
• Dufty, A.M., Jr. 1982. Movements and activities of radio-tracked Brown-headed Cowbirds. Auk 99:319-327.
• Fleischer, R.C. 1985. A new technique to identify and assess the dispersion of eggs of individual brood parasites. Behav. Ecol. Sociobiol. 17:91-99.
• Folkers, K.L.  and P.E. Lowther. 1985. Responses of Red-winged Blackbirds and Yellow Warblers to Brown-headed Cowbirds. J. Field Ornithol. 56:175-177.
• Friedmann, H. and L.F. Kiff. 1985. The parasitic cowbirds and their hosts. Proc. West. Found. Vert. Zool. 2:225-304.
• Friedmann, H., L.F. Kiff and S.I. Rothstein. 1977. A further contribution to knowledge of the host relations of the parasitic cowbirds. Smithsonian Contr. Zool. 235.
• Gates, J.E. and L.W. Gysel. 1978. Avian nest dispersion and fledging success in field-forest ecotones. Ecology 59:871-883.
• Grzybowski, J.A., R.B. Clapp and J.T. Marshall, Jr. 1986. History and current population status of the Black-capped Vireo in Oklahoma. Amer. Birds 40:1151-1161.
• Johnson, R.G. and S.A. Temple. 1990. Nest predation and brood parasitism of tallgrass prairie birds. J. Wildl. Manage. 54:106-111.
• Laymon, S.A. 1987. Brown-headed Cowbirds in California: historical perspectives and management opportunities in riparian habitats. West. Birds 18:63-70.
• Lyon, L.A. and D.F. Caccamise. 1981. Habitat selection by roosting blackbirds and starlings: management implications. J. Wildl. Manage. 45:435-443.
• May, R.M. and S.K. Robinson. 1985. Population dynamics of avian brood parasitism. Amer. Nat. 126:475-494.
• Robertson, R.J. and R.F. Norman. 1976. Behavioral defenses to brood parasitism by potential hosts of the Brown-headed Cowbird. Condor 78:166-173.
• Robertson, R.J. and R.F. Norman. 1977. The function and evolution of aggressive host behavior towards the Brown-headed Cowbird. Can. J. Zool. 55:508-518
• Rothstein, S.I. 1975. Evolutionary rates and host defenses against avian brood parasitism. Amer. Nat. 109:161-176.
• Rothstein, S.I., J. Verner and E. Stevens. 1980. Range expansion and diurnal changes in dispersion of the Brown-headed Cowbird in the Sierra Nevada. Auk 97: 253-267. 
• Sauer, J.R., J.E. Hines and J. Fallon. 2001. The North American Breeding Bird Survey, Results and Analysis 1966-2000. Version 2001.2, USGS Patuxent Wildlife Research Center, Laurel, MD.
• Smith, J.N.M., P. Arcese and I.G. McLean. 1984. Age, experience, and enemy recognition by wild Song Sparrows. Behav. Ecol. Sociobiol. 14:101-106.
• Teather, K.L. and R.J. Robertson. 1985. Female spacing patterns in Brown-headed Cowbirds. Can. J. Zool. 63:218-222.
• Teather, K.L. and R.J. Robertson. 1986. Pair bonds and factors influencing the diversity of mating systems in Brown-headed Cowbirds. Condor 88:63-69.
• Walkinshaw, L.H. 1983. Kirtland's Warbler: the natural history of an endangered species. Cranbrook Inst. Sci. 58.
• Wisconsin Society for Ornithology. 2002. Wisconsin Breeding Bird Atlas.
• Woodward, P.W. and J.C. Woodward. 1979. Survival of fledgling Brown-headed Cowbirds. Bird-Banding 50:66-68.
• Yokel, D.A. 1986. Monogamy and brood parasitism: an unlikely pair. Anim. Behav. 34:1348-1358.
• Yokel, D.A. 1989. Intrasexual aggression and the mating behavior of Brown-headed Cowbirds: their relation to population densities and sex ratios. Condor 91:43-51.