H.J. Ruiz – Avian101

Brown-headed Cowbird

The Brown-headed Cowbird (Molothrus ater) is a small obligate brood parasitic icterid of temperate to subtropical North America. They are permanent residents in the southern parts of their range; northern birds migrate to the southern United States and Mexico in winter, returning to their summer habitat around March or April.

The brown-headed cowbird is typical for an icterid in general shape, but is distinguished by a finch-like head and beak and is smaller than most icterids. The adult male is iridescent black in color with a brown head. The adult female is slightly smaller and is dull grey with a pale throat and very fine streaking on the underparts. 

The brown-headed cowbird is an obligate brood parasite: it lays its eggs in the nests of other small passerines (perching birds), particularly those that build cup-like nests. The brown-headed cowbird eggs have been documented in nests of at least 220 host species, including hummingbirds and raptors.The young cowbird is fed by the host parents at the expense of their own young. Brown-headed cowbird females can lay 36 eggs in a season. More than 140 different species of birds are known to have raised young cowbirds.

Unlike the common cuckoo, the brown-headed cowbird is not divided into gentes whose eggs imitate those of a particular host.

Some host species, such as the house finch, feed their young a vegetarian diet. This is unsuitable for young brown-headed cowbirds, meaning almost none survive to fledge.

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© HJ Ruiz – Avian101