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Brown-headed Cowbird
Molothrus ater
– Family
Authors: Lowther, Peter E.

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Brown-headed Cowbird,adult male.
Fig. 1. Distribution of the Brown-headed Cowbird.

The Brown-headed Cowbird, North America’s best known brood parasite, lays its eggs in the nests of many different species. Originally these “Buffalo Birds” were limited to short-grass plains, where they followed herds of buffalo (Bison bison) and fed on the insects stirred up by their movement. This cowbird has since dispersed widely as European settlement opened forests and homogenized the environment into the agricultural and suburban landscapes of today. Cowbird expansion has exposed naive populations and new species to brood parasitism, and the pressure on such host populations can be substantial. Female cowbirds wander widely, overlap their breeding ranges, and may lay 40 eggs per season.

Cowbirds are difficult to study because their breeding activities are distributed among many host nests. Herbert Friedmann’s monograph (1929) provides a beginning to the vast and sometimes conflicting literature on the Brown-headed Cowbird (see also Friedmann 1963, Friedmann et al. 1977, Friedmann and Kiff 1985). Life history studies of host species also give information on cowbird biology and host-cowbird interactions; see especially studies of Prairie Warblers (Dendroica discolor; Nolan 1978), Kirtland’s Warbler (Dendroica kirtlandii; Mayfield 1960, Walkinshaw 1983), Ovenbird (Seiurus aurocapilla; Hann 1937) and Song Sparrows (Melospiza melodia; Nice 1937). Besides Kirtland’s Warblers, other endangered or threatened species are also “good” hosts, so management concerns require a full understanding of cowbird biology.