Brown-headed Cowbird, Hines, Oregon, 24 May 2009 by Greg Gillson.
Brood parasite: "Organisms that use the strategy... involving the... use of host individuals... to raise the young of the brood-parasite." -- Wikipedia.
The Brown-headed Cowbird does not build its own nest. Instead, it lays its eggs in nests of other birds. In fact, at least 221 known bird species have been hosts to Brown-headed Cowbirds. They usually only lay one egg per host nest. The host parent, often a much smaller species of bird, raises the cowbirds' young, usually to the detriment of its own young.
In "Brown-headed Cowbird: Villain or Scapegoat?" Birding 31:448–451, August 2004, author Stephen I. Rothstein argues that widespread population declines of host species by cowbirds is not scientifically supported. He believes that "North America's most reviled native bird" is unfairly blamed for declines in bird populations largely caused by habitat loss. Of course, we know who is responsible for altering and destroying the established natural landscape, now don't we?
So let's take a step back for a moment and view this bird without prejudice.
The Brown-headed Cowbird is a migratory blackbird. It arrives in the Pacific NW in April and remains into October. A few birds may be found in winter mixed in flocks with other blackbirds at cattle feedlots and similar concentration points.
The male has a black body and brown head; the female is slate-colored gray-brown. The young birds are brownish-gray with paler feather edgings. The bill is pointed and rather thick. Thus, as with female Red-winged Blackbirds, this combination causes some confusion with beginners in thinking juvenile cowbirds might be a sparrow or bunting.
Juvenile Brown-headed Cowbird, Hillsboro, Oregon, 13 July 2010 by Greg Gillson.
Two hundred years ago, this bird was restricted to the Great Plains. They followed great herds of American bison as they traveled, eating the insects kicked up as the huge mammals walked through the grass. Because of the nomadic lifestyle of the bison, cowbirds couldn't stay in one place to raise their young. Thus, the strategy formed of laying its eggs in other bird's nests.
With the subsequent cutting of forests, both in the East and West, the cowbird substantially increased its range. Cowbirds became common west of the Cascades only in the 1960's. In the past 50 years they have increased tremendously in the Pacific Northwest.
Male cowbirds give a rising squeaking call (starling-like) as they chase females in courtship. They also give a bubbly rattle call, similar to Bullock's Oriole.
Besides insects, cowbirds eat seeds. So, you may host cowbirds at your bird feeder--especially, it seems, east of the Cascades.
Note: I write some of my "natural history" articles weeks in advance and have them published on a schedule. As sometimes happens in blogging about birds, another blogger posted an article on cowbirds while mine was in the queue (in news parlance, I was scooped!). Please read Dave Iron's account on Brown-headed Cowbirds that appeared on the BirdFellow blog on June 18.