Parasitic cowbird doesn’t hesitate to take over another bird’s nest

I have to admit there is one bird I’m not glad to see arrive here each spring.

It’s an innocuous-looking bird. The males have a black, iridescent body and dull brown head, and the females are a gray shade of brown. It belongs to the family Icteridae, which includes orioles, blackbirds, grackles, meadowlarks and bobolinks. But it exhibits a behavior pattern not shared by any member of this family.

Brown headed cowbirds, so named because they were often seen in close proximity to bison – and later cattle – are nest parasites, the only one of their kind in North America. The female will lay her egg in the nest of a host bird, most often a songbird. If the host bird does not recognize the egg as not her own, she will be stuck raising a nestling that is huge in comparison to her own brood. Sometimes, the cowbird young will completely crowd out the young of the host bird. Researchers also have reported that mother cowbirds may eject the eggs of the host bird from the nest before she lays her own.

There is some contention as to whether cowbirds affect the percentage of host birds’ reproductive success. Observers such as Donald and Lillian Stokes, authors of the “Bird Behavior” guides, say the survival rate of cowbird young is actually very low. They raise interesting questions about this species, such as how cowbird young recognize their own kind when they’re not raised by them, and what prevents them from becoming attached to their host species.

Others say that the numbers of cowbirds have steadily increased since the clearing of forests for farmland, which has provided just the right habitat for them. Author Scott Weidensaul dubs the female cowbird an “egg factory,” since she may lay more than 40 each season.

Another catalyst for cowbird parasitism is what is called the edge factor. Because large tracts of forest are continually fragmented and reduced to smaller woodlots, deep-forest birds are more exposed to predation from species that prefer the edges of forests. This scenario is one the cowbird has been quick to exploit.

Each species has its own niche in the environment, evolving for a reason. But when other factors such as pollution and loss of habitat affect specific populations, the balance is unevenly tipped in the cowbird’s favor.

There are some things you can do if you notice cowbirds in your area. Do not feed on the ground between May and June. Cowbirds are by nature open-ground feeders, so removing this offering will encourage them to move elsewhere. If you don’t want to lose other ground-feeding birds, you can distribute seed under bushes. Bushes, however, provide good cover for feral and outdoor hunting cats – and this opens up another can of worms.

Chris Corio, a volunteer at Fields Pond Nature Center, can be reached at fieldspond@juno.com.