The Brown-headed Cowbird
(Molothrus ater) is a small icterid.
Adults have a short finch-like
bill and dark eyes. The adult male is mainly
iridescent black with a brown head. The adult
female is grey with a pale throat and fine streaking
on the underparts.
Breeding in open or semi-open
country across most of North America, this bird
is a brood parasite: it lays its eggs in the
nests of other small passerines (perching birds),
particularly those that build cup-like nests,
such as the Yellow Warbler. The young cowbird
is fed by the host parents at the expense of
their own young.
Brown-headed Cowbirds are permanent
residents in the southern parts of their range;
northern birds migrate to the southern United
States and Mexico. They often travel in flocks,
sometimes mixed with Red-winged Blackbirds or
European Starlings.
These birds forage on the ground,
often following grazing animals such as horses
and cows to catch insects stirred up by the
larger animals. They mainly eat seeds and insects.
At one time, the Brown-headed
Cowbird followed the bison herds across the
prairies. Their parasitic nesting behaviour
complimented this nomadic lifestyle. Their numbers
expanded with the clearing of forested areas
and the introduction of new grazing animals
by settlers across North America. Brown-headed
Cowbirds are now commonly seen at suburban birdfeeders.
Brown-headed Cowbird females can
lay 36 eggs in a season. Over 140 different
species of birds are known to have raised young
cowbirds. Host parents may sometimes notice
the cowbird egg. Different species react in
different ways. Gray Catbirds destroy the egg
by pecking it. Some species may simply build
a new layer over the bottom of the original
nest. Brown-headed cowbird nestlings are sometimes
expelled from the nest.
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