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A land manager's guide to conserving habitat for forest birds in ...

A land manager's guide to conserving habitat for forest birds in ...

Brown-headed Cowbird

Brown-headed Cowbird Molothrus ater parasitism rates may be as high as 50 percent, the pressure on some host populations can be substantial. See pages 16 and 25 for additional photos of Brown-headed Cowbirds. Diet Brown-headed Cowbirds depend on open fields, crop and pasture lands for foraging. They feed primarily on weed seeds (grasses and crop grains) and insects (primarily grasshoppers and beetles). Female Brownheaded Cowbird (foreground) with males — Photo: Greg Lavaty Identification (17–22 centimetres) Originally a bison-following bird of the Great Plains, the Brownheaded Cowbird spread eastward in the 1800s as forests were cleared. Notorious as a brood parasite that lays its eggs exclusively in other birds’ nests, it never raises its own young. It is a mediumsized songbird with a stout pointy bill and dark eyes. Males are shiny black with a brown head and neck, while females are dull grey-brown with faint streaking. Conservation Status The Brown-headed Cowbird breeds throughout southern Canada and across the United States. It is often seen near short-grass fields and lawns. Though previously restricted to the short-grass plains, its range expanded widely as European settlement opened forests and created large regions of agricultural and suburban habitat. In Ontario, the distribution of the cowbird is limited by the northern edge of the Canadian Shield. Populations have decreased continentally in recent decades due to the abandonment of farms, urbanization, agricultural intensification, and blackbird control programs. In particular, the disappearance of pastureland over the past 25 years has been implicated in the decline of the cowbird in Ontario. Breeding Biology The Brown-headed Cowbird is a brood parasite that depends on hosts to care for and raise its young. Females are prolific breeders, laying up to 40 eggs each breeding season. Though 89 host species have been documented in Ontario, Cowbirds seem to have a preference for shrub and tree nesters with eggs smaller than their own. Frequent host species include the Red-eyed Vireo, Yellow Warbler, Northern Cardinal, and Song Sparrow. Females tend to exploit forest-field edges, where they typically remove one to two eggs from the nests they parasitize. Once hatched, the cowbird young are reared alongside the hosts, but tend to be larger, more aggressive and faster growing. Because of this unfair advantage, the cowbird may be the only nestling to survive, often suffocating, starving, or ejecting the weaker host young out of the nest. Because Management Guidelines In Ontario, reduction of large, contiguous tracts of forest to small, isolated fragments has benefited the Brown-headed Cowbird at the detriment of a large suite of host species that have had no historical exposure to cowbirds. These species have not evolved mechanisms for rejecting cowbird eggs and are particularly vulnerable to parasitism. As edge specialists, cowbirds respond favourably to all types of forest harvest. Intensive harvesting, such as diameter-limit cuts, particularly result in higher densities of cowbirds. By maintaining higher tree densities (basal area) post-harvest and creating fewer edge habitats, the frequency and intensity of parasitism will decline. As such, efforts should be made not only to follow silviculture standards but to conserve, expand and preserve large, mature forest tracts to benefit host species with small populations. How to find You can spot cowbirds most easily when small groups of males gather to court a single female. Listen for the gurgling sounds of the birds while displaying. Did you know? Brown-headed Cowbird egg in parasitized Veery nest — Photo: Mark Peck • Brown-headed Cowbirds have been known to return to ransack the nests of host species when their egg was removed. • Cowbirds also destroy nests as a type of “farming behavior” to force hosts to build new ones, subsequently laying their eggs in the new nests. • The Brown-headed Cowbird lays eggs in the nests of many different species of birds. Though some female cowbirds will use a number of different hosts, they often specialize on a particular host species. • Some species, like the American Robin and Blue Jay, are cowbird egg “ejectors.” These species can recognize foreign eggs and usually throw any cowbird eggs out of their nest. • Few species can escape Brown-headed Cowbird parasitism, since commutes between feeding and breeding areas often range from two to seven kilometres and up to 15 kilometres. • Fledgling cowbirds eventually leave the host parents and find groups of adult and other juvenile Brown-headed Cowbirds by mid-late July. These flocks can be easily spotted in agricultural fields in late summer. 134 Bird Species Accounts

List of Photographers Al Woodliffe Algonquin Park Museum B.Henry c/o VIREO Bill Hubick Bob McBroom Brad Woodworth Brandon Holden Dan Kaiser Daniel Cadieux Dawn Burke Doug Tozer Eric Boysen F. Truslow c/o VIREO G.K. Peck Garth McElroy Greg Lavaty Harold Lee J.Fuhrman c/o VIREO Jarrid Spicer Jayne Gulbrand Jeff Nadler Ken Elliott Kyle Aldinger Larry Watkins Lindsay MacLean-Abbott Lucas Foerster L. Walkinshaw c/o VIREO Marie Read Mark Marek Mark Peck Michael Patrikeev Nick Bartok OMNR Peter Burke R.Curtis c/o VIREO Robert McCaw Scott Gillingwater Scott Reid Teresa Piraino Terry John Myers Terry Schwan Terry Sohl Thomas Whelan Visual Resources for Ornithology (VIREO) W.Greene c/o VIREO Trout Lily — Photo: Robert McCaw List of Photographers 135

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