One of the many birds returning to Maine for the breeding season is the Brown-headed Cowbird.    The male is a striking bird with a brown head on a sleek black body.  The female is a drab gray. Cowbirds feed on items as varied as insects (especially grasshoppers), seeds and grasses.

The Brown-headed Cowbird is a relatively recent arrival in eastern North America.  Before Europeans colonized the east, forests were the general rule in the landscape.  Cowbirds were restricted to the west where they followed the huge bison herds, feeding on insects be disturbed by the bison.  Cowbirds are much more likely now to feed among flocks of cows rather than bison, hence the common name.

Cowbirds have had an insidious negative impact on many species of songbirds in North America.  Cowbirds are brood parasites, meaning that females lay eggs in the nests of other species of birds.  Host parents are tricked into accepting a cowbird egg as one of its own.

Cowbirds are called obligate brood parasites because they rely totally on other species of birds to do their parenting.  Cowbirds never form nests, brood their eggs or feed their nestlings.

The embryo inside a cowbird egg develops at a faster rate than the host’s eggs.  A female cowbird retains an egg in its uterus for about 18 hours before it is laid, giving the developing chick a head start.

Since the cowbird chick hatches before the eggs of its hosts, the cowbird chick gets all of the food brought by the host parents until the other chicks hatch.  By then, the cowbird chick has a size advantage and succeeds in getting more than its share of the food brought by the host parents.  In some cases, cowbird chicks are known to push the unhatched eggs out of the nest by instinct; the cowbird chicks are blind when they push the eggs out of the nest.

Cowbirds parasitize over 200 species of breeding birds in North America.  Sometimes, the prospective host parents detect the presence of the cowbird egg and will remove the egg or abandon the nest.  Lots of host species can’t distinguish the cowbird eggs and accept them readily.  In part, this lack of discrimination arises because cowbirds have only been present in the east for the last 150 years.  Host species have not had time to evolve the ability to discern a cowbird egg among their own eggs.

It is heart-rending to see a pair of Yellow Warblers feeding a cowbird chick that is larger than the host parents.  But, we have to remember that nature is not always kind.  Brood parasitism represents an effective way for one species to perpetuate itself at the expense of other species.

[First published on April 29, 2012]