Project NestWatch - Brown-headed Cowbird fact Sheet

Species description:	The male adult is black with a brown head while the female is all gray-brown. Juveniles look like females but are also streaked below. The average adult weight is 44 g.
Nesting behaviour:	This species does not build a nest nor does it incubate its eggs. It is known as a parasitic species: it lays in other birds' nest, often removing an egg from that nest at the same time. When food is abundant in the environment, female Cowbirds can lay almost daily throughout the nesting season. At the pace of about one egg per day, a single female can lay as many as 40 eggs in the same season. These eggs are usually added to nests when the other bird has not laid all of its clutch.
Impact of Cowbirds on other bird species:	Many birds are parasitized by the Cowbird, the most common species being the Yellow Warbler, the Song Sparrow, the Red-eyed Vireo, and the Chipping Sparrow. The literature reports that Cowbird eggs have been found in nests of over 220 species across North America, mostly passerines. Of these, about 145 species have been known to rear cowbird young. Other species, like the American Robin and Blue Jay, usually throw out of their nest any Cowbird's egg, although a small percentage of these birds' nests have been found containing one.
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.
Map produced with digital range map files provided by the Canadian Wildlife Service. To view maps for additional species, visit http://wildspace.ec.gc.ca/maps-e.html
Nesting habitat:	Cowbirds usually lay in nests that are found in an environment with low or scattered trees among grassland vegetation, including prairies, pastures, orchards, woodland edges, etc. Cowbirds often avoid dense forests and extensive field or prairie areas. Scientific studies have shown that Cowbirds prefer to lay in nests that already have 2 eggs and in which the host's eggs are smaller than their own.
Egg colour:	White to grayish white, marked with brown streaks. Often, these markings are heavier at the large end of the egg.
Parental care:	None. As Cowbird eggs are laid in other bird's nests, the eggs and young are cared by the "adopting" adults.
Young:	As Cowbird eggs develop rapidly, they are often the first ones to hatch in a nest. The young are fed by the adults of the nest in which it was incubated. A young Cowbird leaves the nest at the age of 8-13 days and will be independent by the age of 25-39 days.
Diet:	Mostly seeds and arthropods (insects). Cowbirds forage on the ground. They eat mostly weed seeds but include grasshoppers and beetles in their diet.