Brood Parasites

Why are we interested in Cowbirds?

Researchers believe that parasitism by cowbirds may be a significant factor contributing to the declining numbers of many songbirds in North America. This is especially problematic in areas of the United States where large tracts of forest are being fragmented for development and timber harvest. This creates forest “edge”—areas where open fields or lawns meet dense woodland. Nest parasites such as cowbirds thrive in such areas, where they have easy access to other bird’s nests.

Cowbirds were historically open-country birds, associating with buffalo herds and later adapting to domestic cattle. The prairies and plains were the cowbirds’ homeland, but they have now expanded their ranges. Currently the Brown-headed Cowbird’s range includes all of the 48 contiguous states and southern Canada, while the Bronzed Cowbird expanded its range out of Mexico and is resident in much of the southwestern U.S.

Historically, Brown-headed Cowbirds laid their eggs in the nests of grassland species. Because these species evolved side-by-side with cowbirds, many of them developed defenses against cowbird parasitism—some can recognize cowbird eggs and eject them from their nests. Most forest-nesting birds, however, have had no long-term exposure to cowbirds. Once a cowbird lays its eggs in the nest of a thrush or other woodland species, the host birds often raise cowbird young rather than their own offspring. Of 220 species known to have been parasitized by Brown-headed Cowbirds, and 144 have raised young Brown-headed Cowbirds successfully. Bronzed Cowbirds, which have a much more limited range in the U.S., are known to parasitize 82 species, and 32 have successfully raised Bronzed Cowbird young. BFL participants can help us learn more about how cowbirds interact with forest-nesting birds.

Territorial behavior and courtship

The type of territorial behavior used by a Brown-headed Cowbird depends on the type of habitat in which it lives, as well as on the sex ratio of males and female in the local cowbird population. In the open farmlands of the Midwest, where host species are less numerous and more concentrated at field edges, Brown-headed Cowbirds do not seem to be territorial. Instead, small flocks of males remain in fixed areas while female home ranges are rather large. A common sight in one of these flocks is the males, either perched or on the ground, displaying to each other. Through these displays, males are competing for dominance in that area. As females roam widely in search of suitable nests in which to lay their eggs, they will mate with the dominant male in each area. Thus, each female may mate with a number of males, resulting in promiscuous matings. In some areas, the home ranges of females are thought to overlap, because eggs of more than one female are frequently found in a single host’s nest. A single cowbird will generally lay only one egg per nest.

In mixed deciduous woodlands where host species are numerous and widely dispersed, cowbirds are thought to be territorial and monogamous. The females return in early spring and form territories from 10–50 acres (4–20 hectares). Males compete with each other for dominance in a female’s territory, and the dominant male then “guards” the female by following her wherever she goes in her territory. When other males come near, he gets between the intruder and his mate and displays, which usually makes the other male leave. The dominant male guards the female because she will mate with whatever male she is near. Female cowbirds will keep other females out of their territories at least during the morning hours, which is when egg laying is accomplished. Later in the day, all cowbirds in a given area feed together at certain prime feeding spots, whether these exist in a bird’s territory or not. Males may continue guarding a mate from other males, but they do not defend a territory. Courtship behavior continues through the egg-laying phase, which stops about mid-summer. Though the Bronzed Cowbird has not been studies as thoroughly as the Brown-headed Cowbird, it is presumed the Bronzed Cowbird’s behaviors and physiology are similar to the congeneric Brown-headed Cowbird.