My fourth grade teacher assigned each student in the class a bird to research. We then wrote a small essay and drew a picture of our bird to share with the class. My bird was the brown-headed cowbird. What a disappointment! I wanted the cardinal, blue jay or another “pretty” bird, not the dull cowbird. As an eight year old I simply did not appreciate how interesting, and colorful in its own way, brown-headed cowbirds (Molothrus ater) are.
About 1% of bird species worldwide are obligate brood parasites, meaning they do not build nests at all. Rather these brood parasites, of which the brown-headed cowbird is a prime North American example, lay their eggs in the nests of other species. All parental duties are performed by the host birds. Rather than spend time and resources building nests and raising their young, female cowbirds produce up to 40 eggs a year. Usually a female cowbird lays only one speckled egg in a host’s nest, however, more than one female cowbird may lay an egg in a particular nest. Brown-headed cowbirds are not monogamous and both males and females may have multiple mates in a season.
When the female brown-headed cowbird lays an egg, she will often push one of the host’s eggs out of the nest, either at the time the egg is laid or by returning to the nest shortly afterward. Thus the host bird(s) do not notice a difference in egg number. Most hosts do not recognize the foreign egg. Those that do detect the “intruder” may build another nest directly on top of the first nest or desert the nest and begin the process anew, may push the offending egg out of the nest or may puncture the alien egg. Cowbird eggs have been found in the nests of over 220 different avian species from hummingbirds to raptors. Some research has found that brown-headed cowbirds tend to use nests with eggs that are smaller than theirs, but that is certainly not always the rule.
Also called a common cowbird, the brown-headed cowbird is native to the open grasslands of middle America, but their range has spread and now cowbirds can be found throughout all of North America except the Arctic. They inhabit pastures, fields, meadows, forest edges and even urban lawns.
Brown-headed cowbirds are blackbirds with stocky bodies, short tails and short, thick-based beaks that resemble the beaks of finches. The eyes are dark and the legs and bills are black. The male brown-headed cowbird has a glossy black body with greenish iridescence and a brown head. Females are a dull brownish-grey with darker wings and tail and fine streaking on the belly.
Cowbirds can usually be found foraging on the ground, often in the company of blackbirds, starlings or other species. They primarily eat the seeds of grasses and weeds or grain crops, supplementing their diet with insects. The common name derives from the fact that cowbirds can often be seen following livestock (originally bison) eating the insects, particularly grasshoppers, that the grazers stir up. Female cowbirds also require calcium to produce large numbers of eggs. This need is met by eating the eggs or shells from host nests.
These cowbirds were photographed in the yard of a friend in Cassel CA (Shasta County).
More on these fascinating birds in my next post. . .