Brown-headed cowbird - a brood parasite

Brown-headed cowbirds lay their eggs in the nests of other birds.

Brood parasitism (also called nest parasitism in birds) occurs when one species relies on another species to raise their own offspring.  Such is the rare behaviour of Brown-headed cowbirds.  A female will find the nest of another species, often a smaller bird, and lay her egg in with the existing clutch.  The kicker here is that females may lay up to 10 eggs, and often lay only one egg per nest.  

The egg itself is larger than the other eggs; the host bird may not recognize it as being foreign.  Some birds, such as yellow warblers, may reline a nest with a cowbird egg and start over.  It was once observed that a yellow warbler built 6 nests, each atop the other, to avoid being victimized in such a manner.

The terrible thing is that the cowbird chick is larger than its foster brothers and sisters and will eject them from the nest.  It also competes for food brought by the parents which may cause the others to starve if they do remain.  This often leaves the cowbird as the sole occupant of the nest and thus prevents the successful breeding of the parasitized pair.  

Cowbird parasitism has been responsible for the decline of quite a few bird species.  The birds' original range was limited to the west where cattle and buffalo roamed, but man's alteration of the environment has allowed them to expand throughout most of North America.  The birds have been responsible for major declines in some eastern woodland bird populations.

Below I have photos (taken years earlier) of a male cowbird (left) with a red-winged blackbird in the background and a buffalo (right) with a pair of cowbirds on its back.  Cowbirds will sit atop a ruminate and wait as the animal feeds and moves about.  When insects are disturbed by these lumbering actions and flee the area, the cowbird catches them and then returns to its furry perch.

Brown-headed cowbird (front, left) and atop a grazing buffalo (right).

Although cowbirds are raised in isolation from others of the same species, they seem to be able to find each other and procreate.  I think that's amazing.

Thanks for reading. 

Eric Svendsen     www.ericspix.com