By Susan Shea
Black birds with a greenish sheen and brown heads sometimes visit my yard during spring migration. These are male brown-headed cowbirds, and they often arrive in mixed flocks of red-winged blackbirds and grackles.
Cowbirds breed in most of the Northeast and have an unusual reproductive strategy. Instead of building their own nests, cowbirds lay their eggs in other birds’ nests and depend on those birds to raise their young. This evolutionary strategy is called brood or nest parasitism. This type of parasitism exists in several different avian groups, but only 1% of bird species are interspecific brood parasites — meaning they lay eggs in nests of other species. Some birds are intraspecific brood parasites, laying their eggs in nests of the same species.
The dull gray-brown female cowbirds deposit their eggs in a variety of nests, ranging from cup to dome-shaped to tree cavities. Their hosts vary in size, but usually have smaller eggs, and include yellow warblers, song sparrows, and red-winged blackbirds. The female cowbird watches other birds construct their nests, and after eggs are laid there, she sneaks in to lay one of her white, speckled eggs. Often, the cowbird ejects an egg from the host nest at the same time. The next day she may lay another egg in a different nest. Cowbirds may parasitize 30 to 40 nests over the course of their two to three-month breeding season. They have been implicated in the decline of some songbird species, such as the endangered Kirtland’s warbler in Michigan.
Free of the responsibility of defending a nest, incubating eggs, and feeding young, cowbirds have more time and energy to forage, mate, and produce many eggs. There are no advantages to the host parents. If they abandon their nest, they’ve wasted the time and energy it took to build it and lay eggs. If they keep their nest, the hosts’ young must compete with the larger cowbird nestling for food and may even be killed by the cowbird chick if food is scarce. As a result, hosts have evolved defenses against nest parasitism. They choose nest sites that are difficult to parasitize and vigorously defend their territories. Victims may eject the cowbird egg if they can manage it or construct a new nest on top of the existing one.
In a co-evolutionary arms race, brood parasites attempt to mimic the eggs of their hosts. Most often, cowbirds deposit their eggs beside other speckled eggs. However, in some cases, cowbird eggs are dissimilar to those of the host and are obvious to the human eye. Surprisingly, host birds sometimes tolerate these eggs. One theory that explains this behavior postulates that hosts put up with the extra work of raising a baby cowbird because they’ve previously experienced retaliation when they’ve removed a cowbird egg. Cowbirds and other brood parasites have been known to destroy a host’s clutch following rejection of their eggs.
Centuries ago, cowbirds followed bison on the Great Plains, feeding on insects the buffalo stirred up. Today they associate with cattle for the same reason — hence the name “cowbird.” Scientists used to believe that because cowbirds travelled with bison herds, they were unable to remain in one location to raise young and therefore took advantage of other birds’ nests. However, a study of cowbird DNA has found that this species was parasitic long before it began following buffalo. Once found only in midwestern grasslands, cowbirds have expanded their range eastward, and their numbers have surged as humans have cleared forests for agriculture and development.
Yellow-billed and black-billed cuckoos are other interspecific brood parasites that inhabit the Northeast. These species practice “facultative” nest parasitism, whereas the cowbird is an “obligate” parasite. Cuckoos construct their own nests, but in some years lay additional eggs in other birds’ nests. This occurs most often in years of plentiful food, such as outbreaks of tent caterpillars, and appears to be a strategy to increase reproductive success by taking advantage of abundant resources. Yellow-billed cuckoos mainly parasitize the nests of black-billed cuckoos, and vice versa.
Several duck species, including wood ducks, are intraspecific brood parasites, distributing extra eggs in the nests of other ducks of the same species. Through this strategy, they reduce the risk of losing all their reproductive effort if their own nest is destroyed by a predator.
These fascinating intrigues and deceptions reveal there is more going on in the world of birds than meets the eye.
Susan Shea is a naturalist, writer, and conservationist based in Vermont. Illustration by Adelaide Murphy Tyrol. The Outside Story is assigned and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of the New Hampshire Charitable Foundation: nhcf.org.
