The Brood Parasite

In urban as well as rural areas across the Finger Lakes Region, there is a parasite looking to strike at any moment. This parasite is incredibly fast-growing, smart, and sneaky. It is the brown-headed cowbird. How can a bird be a parasite, you ask? This bird is a brood parasite, meaning it does not build a nest and care for its own young, rather female brown-headed cowbirds lay their eggs in the nests of other birds and allow the host to raise their babies.

The brown-headed cowbird is a wide-spread songbird common across the United States. It prefers disturbed habitats, forest edges, and fields. Male birds of this species have shiny black bodies with brown heads, hence the name. They display by spreading their wings and tail, dipping their head down, and bursting out with a song that sounds like a few notes of a babbling brook followed by a high-pitched squeak. Females are gray and very nondescript, the better to camouflage themselves with.

During the nesting season, female brown-headed cowbirds steal into the unattended nest of another bird species, lay an egg within minutes, and fly off. Before leaving, she may damage or remove another egg in the nest. This species is known to parasitize the nests of as many as 220 different species of songbirds and one female is able to lay 40 or more eggs a single season.

Cowbird eggs develop quickly and usually hatch prior to those of the host species. Nestlings then grow rapidly, fledging within 10-11 days. During this time, the host parent will feed and care for the young cowbird even though it is much larger than its own young and may even out-compete the host chicks. As odd as it looks, the young brown-headed cowbird is often by much larger than the host parent by the time it fledges.

Brown-headed cowbirds are known to parasitize nests of some bird species with endangered or threatened conservation status which is a concern, but alleviation of habitat loss, migration route degradation, and reduction of losses due to pesticide and herbicide poisoning have shown to be better indicators for positive conservation measures rather than removing the eggs of parasitic bird species. Please remember, brown-headed cowbirds are protected by the Migratory Bird Treaty Act and it is unlawful to remove eggs or use lethal measures against them even with the intention of helping a vulnerable species. Actually, if you look at it the right way, brown-headed cowbirds are not malicious but have come up with a brilliant reproductive method using the least amount of energy input. Hmmm, that gives me an idea…


Story and photo by Gabrielle L. Wheeler