Nature's sneaky obligate brood parasites: Cowbirds lay eggs and run

"All God's creatures got a place in the choir

"Some sing low and some sing higher … "

Those are the first lines of a song written in 1979 by Bill Staines, once a regular on Garrison Keillor's radio show.

The brown-headed cowbird, our resident cowbird, does have a place in the choir, even if the bird is scorned for its behavior. Sometimes, with sharp elbows, so to speak, it pushes other native species right off the stage.

Cowbirds are obligate brood parasites, obligated by nature to lay their eggs in the nests of other bird species. This is done on the sneak. Cowbirds thus avoid the chores of parenthood. This causes problems for the unwitting host.

Brown-headed cowbird eggs have been found in the nests of 221 North American songbird species.

There are six cowbird species in the Western Hemisphere, three in the U.S. — brown-headed, bronzed and shiny.

Ninety-six known bird species in the world — about 1% — are obligate brood parasites. They include African honeyguides, about half of the cuckoo species, the black-headed duck in South America, and cowbirds.

"Heavy parasitism by cowbirds has pushed some species to the status of 'endangered' and has probably hurt populations of some others," according to information on the Audubon website.

Notable here is the Kirtland's warbler, its small population endemic to the Upper Peninsula of Michigan with a few recently reported nests in Wisconsin. The Michigan birds are kept extant only by vigorous trapping of cowbirds.

Cowbirds feed at the feet of grazing animals, eating insects flushed by hooves. Buffalo hooves did the job before settlers put the plow to that territory. Cattle then became prime providers.

We helped by opening the landscape, making more land attractive to the birds.

Cowbirds favor open, cuplike nests. Some of the appointed hosts learn to recognize cowbird eggs and remove them from the nest. Other species build a second nest atop the eggs. And many don't do either.

"Song sparrows happen to have eggs very similar in size and spotting pattern to those of the cowbird, and almost invariably raise the cowbird young," according to information at web.stanford.edu.

It adds, "In contrast, catbirds and robins, which lay unmarked blue eggs, almost invariably eject cowbird eggs from their nests."

Gray catbirds are another exception, according to Audubon. Catbirds instinctively learn the look of the first egg in their nests, almost always theirs. If later eggs do not match, out they go.

Studies have shown that as many as 50% of catbird nests are targeted by cowbirds, yet it is rare for a catbird to raise a cowbird chick.

Cowbirds are larger than most host birds, easily double the size of sparrow and warbler species. Cowbird chicks grow fast, mature early, dominate nests, command food. Host chicks are neglected.

Audubon predicts that a warming climate will expand cowbird range deep into Canada and Alaska.

There is a note of hope, according to Audubon: "If a host species changes its breeding behavior — arrives at nesting sites earlier, for example, or begins nesting in a new location — the parasite must keep up.

"If it doesn't, there may be no [host nests for its] offspring, and without any parenting skills, it may fail to reproduce.

"While avian parasites and hosts have endured climate swings before, never have they faced such rapid warming," the Audubon website says.

Lifelong birder Jim Williams can be reached at woodduck38@gmail.com.