Woodland Birds, Pacific Northwest
The Brown Headed Cowbird is a favorite bird of mine even though most people do not like them. The brown headed cowbird is one of two species of cowbirds found in North America. Both species are brood parasites and lay their eggs in the nests other birds.
Formerly occurring in the central grasslands of North America, wholesale clearing of forested land has allowed the brown headed cowbird to extend its range across most of North America, and to increase its population dramatically.
The male has an iridescent black body and a brown head. The female is slightly smaller than the male and uniformly gray.
The coffee brown head of the male is a distinguishing field mark. The strong conical bill is adapted to its diet of insects, seeds and berries. The egg of the brown headed cowbird is white, bluish or greenish with a fine speckling of reddish brown.
Brown Headed Cowbirds never build nests or raise their own young. Males typically arrive on the BC coast before the females who arrive a week or so later. Females will lay eggs in other birds’ nests and leave the rearing to them. They find nests to lay eggs in by looking for birds building nests, either by sitting quietly perched in a tree and watching for birds going in or out of a nest or watching other birds gather nesting material and following them. The female generally chooses a nest to place her eggs in by its shape, and will lay one egg per nest. She waits to lay the egg until there is one or more eggs in the nest, and then removes an egg from the nest replaces it with her own. She continues this over a period of about a month and she can lay up to 40 eggs a season. Incubation time is short, up to 12 days, this gives the young cowbird a head start in the nest.
Young cowbirds grow rapidly, giving them a competitive advantage over the other young in the nest, they will quite often push the newly hatched birds out of the nest. Young cowbirds usually leave the nest in about 13 days, but are not fully independent from their hosts until they are about a month old.