Brown-headed Cowbirds are notorious for the unusual approach to raising their young, or the lack thereof. Known as brood parasites, cowbirds lay their eggs in the nests of other birds. These unsuspecting “foster parents” are called hosts, and will usually raise young cowbirds at the expense of their own. Talk about an aviary freeloader.
Male Brown-headed Cowbirds sport glossy black plumage with a green sheen and a dark-brown head. Females are much less showy, with dull grayish brown feathers. The bland appearance of the female allows them to observe possible hosts building nests without drawing suspicion. Once the location of a good nest is known, the cowbird will return during the host species egg-laying period in hopes it will go unnoticed. Because cowbird eggs typically hatch earlier than those of the host, the cowbird has an advantage, in more ways than one.
In most parasitized nests, the Brown-headed Cowbird removes one the of host eggs before laying its own to ensure it is incubated. Once hatched, the fledging continues to severely affect the host family. As cowbirds are usually quite larger than the host specie, the fledgling can dominate over nest-mates in space occupied and during feeding. They develop faster too, sometimes nudgling out eggs and smaller fledglings to give them more room. While the impact varies for different host species, what usually occurs is the loss of at least one of the host's young.
More than two hundred species are known to be parasitized by Brown-headed Cowbirds. While there are those that known to evict cowbird eggs and fledgling for its nest, many species end up hatching the somewhat lazy bird’s young.
June 15, 2011