Description
Slightly smaller than a red-winged blackbird, the brown-headed cowbird (Molothrus ater) is a stocky blackbird with a distinctive short tail. Males have an iridescent black body with a deep brown head that looks black in certain lights. Females are completely brown. This bird’s thick triangular bill is reminiscent of a finches’ beak.

Habitat and Range
Originally residents of the midwestern grasslands, cowbirds take their name from their habit of following bison and cattle to capture the insects the herd stirred up. Due to urban sprawl and the resulting loss of woodlands, brown-headed cowbird populations have rapidly expanded into the eastern U.S. Today the bird is often found in open areas such as fields, pastures, parks and lawns. Short distance migrants, brown-headed cowbird often join winter flocks made up of thousands of blackbirds.

Behavior
During the breeding season, males gather to put on displays to attract the attention of females, who remain busy patrolling woodland edges for other birds’ nests in which to lay eggs. As one might expect from birds without a nest, cowbirds are not necessarily monogamous, and may have several mates in one season.

Song
Brown-headed cowbirds are noisy creatures, with a variety of whistles, clicks and calls. Listen to their sounds here.

Feeding
In addition to insects, cowbirds primarily eat grass and weed seeds. They frequently forage in mixed-species flocks, along with blackbirds, grackles and starlings. Due to their constant egg-laying, females may eat snail shells or even the eggs of other birds to restore their calcium levels.

Backyard feeding stations can attract the cowbird, particularly if seeds are scattered on ground. Despite the fact that the brown-headed cowbird is a native species, its parasitic nesting habits lead some bird lovers to regard it as an unwelcome visitor.

Parasitism
Studies have shown that brown-headed cowbirds take advantage of the nests and parenting skills of hundreds of other bird types. Most commonly, cowbird hatchlings end up being raised by song and chipping sparrows, eastern and spotted towhees, yellow warblers, red-eyed virios and red-winged blackbirds. Genetic analysis reveals that individual cowbird females, which can produce as many as three dozen eggs per summer, tends to specialize in a particular species.

Not only does the female cowbird lay her eggs in another bird’s nest, she may even damage eggs that are already there. The species also has the advantage of fast-developing eggs. This allows the baby cowbirds a greater share of food from its foster parents. Cowbirds are aggressive even when newly hatched, when they may roll any rival eggs out of the nest.

Some species actually recognize cowbird eggs and take action to prevent their hatching. Too small to remove the usurper’s eggs from its own nest, the yellow warbler builds a new nest on top. Other, larger birds may puncture the shells of cowbird eggs, or throw them from the nest. Most species, however, don’t differentiate cowbird eggs from their own, and raise the subsequent hatchlings as their own.


Photo from Creative Commons by Kelly Colgan Azar