Description 6 - 8" Males are black with glossy brown head; females plain
gray-brown. Both have a finch-like bill. Juveniles resemble the adult female.
Habitat Agricultural land, fields, woodland edges, and suburban areas.
Diet During the summer, about half of the Brown-headed Cowbirds' diet is made up of
seeds, and the other half consists of insects and other invertebrates. Their winter diet is
made up of more than 90% seeds and waste grain.
Females also eat eggshells from the nests where they lay their eggs and mollusk
shells, especially during the breeding season. This practice may provide the calcium
they need to produce the large number of eggs they lay.
Nesting Brown-headed Cowbirds have unusual breeding behavior: they never build
nests or raise their own young. Cowbirds are brood parasites and promiscuous; no
pair bond exists. Males typically arrive on the breeding grounds before the females.
In late spring the female cowbird arrives to several suitors who have moved into the
woods. The males sit upright on treetops, uttering sharp whistles, while the female
searches for nests in which to lay her eggs. Females lay their eggs in other birds' nests
and leave the rearing to other species.
They find nests to parasitize by looking for birds building nests, either by walking along
the ground, perching quietly in shrubs or trees, or making noisy flights back and forth,
possibly to flush potential hosts.
The female generally chooses an open cup-nest to parasitize, and usually lays one
white egg, lightly speckled with brown, per nest. She waits to lay the egg until the host
bird has at least one egg in its nest, and often removes one egg from the nest before
laying her own. She continues the process over a period of about a month and can lay
up to 40 eggs a season.
Incubation time is short, 10 to 12 days, which allows the young cowbird to get a head
start in the nest. Young cowbirds grow rapidly, giving them a competitive advantage over
the other young in the nest.
Young cowbirds usually leave the nest after 8 to 13 days, but are not fully independent
from the hosts until they are about 25 to 39 days old. Then they typically form small
flocks with other juveniles.
Over 220 species have been observed with Brown-headed Cowbird eggs in their nests,
and at least 144 species have raised Brown-headed Cowbird young to the fledgling
stage, often at the expense of the young of the host, pushing them out of the nest or
taking most of the food.
Range Breeds from British Columbia, central Saskatchewan, central Ontario,
Quebec, and Newfoundland southward throughout United States except extreme
Southeast and Florida. Winters in central and southern part of breeding range as well
as in Florida.
Brown-headed Cowbirds are typically short- to medium-distance migrants, but may be
present year-round in some parts of their range. They often migrate in flocks with other
Voice Squeaky gurgle. Call is check, a rattle, or a whistle. Cowbirds can sing notes
so high that the human ear can't even hear them.
Discussion It has been suggested that cowbirds became parasitic because they
followed roving herds of bison and had no time to stop to nest.
The courtship ritual has been most comical to watch. The male will hunch his 'shoulders' and dance by bobbing up
and down on his legs in a showy display to the female. Once the bobbing begins, he then starts singing the
strangest garbley-warbley song you've ever heard. Though she watched every meticulous movement he made, to
my human eyes, she appeared unimpressed.
|April 2006 Female and Male