Brown-headed Cowbird

Molothrus ater

Brown-headed Cowbirds, North America's best known brood parasites, are easily sighted. They often perch in full view presumably to court mates and select host nests. Because of the glossy blackness of their plumage and their brown heads, adult males are easily recognized. Female cowbirds are smaller, dusty brown and drab, and less easily identified. Brown-headed Cowbirds emit a variety of easily-recognized calls, from liquid "glugs" to squeaks and whistles.

Code Frequency

Due to their conspicuous habits, Brown-headed Cowbirds were easy to detect wherever they occurred. Certain codes such as nest building and food for young are inappropriate for obligate brood parasites. Atlasers accomplished confirmation of breeding only by the observation of a cowbird egg or young in another bird's nest, by the observation of a female cowbird visiting a nest, or by the observation of a fledgling cowbird being fed by a host species. Except for host species feeding of Brown-headed Cowbird young, confirmation of breeding was difficult for this species. Additionally, Brown-headed Cowbirds will commute up to seven kilometers between foraging and nesting territories (Robinson et al. 1993), and commuting birds may have accounted for some records in which breeding was not confirmed.

Distribution

As expected, Brown-headed Cowbirds were distributed essentially statewide, although they were not recorded at all in many adjacent blocks in Reynolds and Shannon counties. This may have been due to the lack of crop fields and pastures in this region. Thompson (1994) postulated that a lack of forest edge limits a cowbird's ability to find hosts. Also, Brown-headed Cowbirds are likely less numerous in areas that lack foraging habitat. Therefore, cowbirds may have avoided the more solidly-forested regions. They were less frequent in the Mississippi Lowlands, which has fewer domestic livestock and grazed habitats.

Average Number of Birds / 100 stops

Abundance

There was an unexpected lack of variation in abundance statewide as determined by Miniroute and Breeding Bird Surveys. Slightly greater numbers were recorded in the Big Rivers and Ozark Border natural divisions than in the remaining four natural divisions.

Phenology

In the wild, Brown-headed Cowbird females ovulate and lay up to 40 Eggs: during their two-month breeding season (Friedmann and Kiff 1985). Atlasers observed nests with cowbird Eggs: from April 24 through August 4. The FY records in the phenology chart are observations of adults of several host species feeding young cowbirds.

Notes

Brown-headed Cowbird parasitism was recorded in 46 host species (Appendix E). Lowther (1993) listed 144 species that have reared cowbird young. He reported that the 15 most parasitized species in North America, beginning with the most frequently parasitized, were Yellow Warbler, Song Sparrow, Red-eyed Vireo, Chipping Sparrow, Eastern Phoebe, Eastern Towhee, Ovenbird, Common Yellowthroat, American Redstart, Indigo Bunting, Yellow-breasted Chat, Red-winged Blackbird, Kentucky Warbler, Willow Flycatcher and Bell's Vireo.

Breeding Evidence

Reported in 1,138 (94.3%) of 1,207 blocks

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