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Brown-headed Cowbird (Molothrus ater)
Fact Sheet

Species Description

  • Small blackbird with a short, thick bill
  • Adult males are black with a brown head; females are gray-brown with fine streaking on undersides
  • Juveniles similar to females but distinctly streaked below
  • Average adult weight is 44 g
Male Brown-headed Cowbird (Photo: Alan Vernon)
Female Brown-headed Cowbird (Photo: Jason Matthews)

Nesting Behaviour

This parasitic species does not build a nest, nor does it incubate eggs. It lays in other birds' nests, often removing an egg from the host nest at the same time. When food is abundant in the environment, female cowbirds can lay almost daily throughout the nesting season (as many as 40 eggs in one season). These eggs are usually added to nests when the other bird has not yet laid a complete clutch. Research has shown that cowbirds prefer to lay in nests that already have two eggs, and in which the host's eggs are smaller than their own.

Impact on Other Birds

Many species (especially songbirds) are parasitized by Brown-headed Cowbirds. Some of the most commonly parasitized species include Yellow Warbler, Song Sparrow, Red-eyed Vireo, and Chipping Sparrow. Cowbird eggs have been found in the nests of over 220 species across North America. Of these, 144 species are known to have reared cowbird young.

Some species, such as American Robins and Blue Jays, often eject cowbird eggs from their nests, although a small percentage of these birds' nests have been found with a cowbird egg.

Nesting Distribution in Canada

The Brown-headed Cowbird lays eggs in nests across Canada. It is absent from Labrador, Nunavut, and most of the Northwest Territories and Yukon.

Nesting Habitat

Cowbirds usually lay in nests found in areas with low or scattered trees among grassland vegetation, including prairies, fields, pastures, orchards, residential areas, and woodland edges. They usually avoid dense forests and extensive open field or prairie areas.

Eastern Phoebe nest with a Brown-headed Cowbird egg (Photo: Galina Osmolovskaya)

Egg Colour

White to grayish white, marked with brown or gray spots. Markings are often heavier at the larger end of the egg.

Parental Care

None. Cowbird eggs are laid in other birds' nests. The eggs and young are cared for by the 'adopting' adults.

Young

Brown-headed Cowbird eggs develop rapidly, and are often the first ones to hatch in a nest. The young are fed by the adults of the nest in which they were incubated. A young cowbird leaves the nest at the age of 8-13 days and is independent by 25-39 days.

Diet

Brown-headed Cowbirds forage on the ground, eating mostly weed or grass seeds. They also eat insects, mostly grasshoppers and beetles.

Sources of Information

All about Birds: http://www.allaboutbirds.org/guide/brown-headed_cowbird/id

BNA online: Lowther, Peter E. 1993. Brown-headed Cowbird (Molothrus ater), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu/bna/species/047 doi:10.2173/bna.47

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