Cool critters: Why the brown-headed cowbirds leave their young to be parented by other birds

Brown-headed cowbirds are hard to love. They aren’t pretty or melodious. And worst of all, they’re deadbeat parents, tricking other birds into rearing their young.

Many people regard them as villains. But from a scientific point of view, they are innovative survivors.

“They’ve been forced to adapt in order to survive,” said Washington State University wildlife ecologist Rod Saylor, an expert on avian behavior and habitat. “It’s an intriguing reproductive strategy that scientists are trying to better understand.”

Native to the United States and widespread across the Inland Northwest, brown-headed cowbirds lay their eggs in the nests of other bird species, leaving it to the new host or “foster” parents to raise their cowbird youngsters. This tactic is called brood parasitism, and it occurs among a variety of birds, fish and insects.

But it seems the brown-headed cowbird excels at it, outsourcing the parenting work to more than 220 bird species, including the smaller-sized sparrows, grosbeaks and barn swallows.

Unfortunately, the strategy often comes “at the expense of at least some of the host’s own chicks,” according to the Cornell Lab of Ornithology. Because the foster parents don’t seem to notice they’re raising an imposter, the larger, noisier cowbird youngsters get most of the food. Consequently, host nestmates can become malnourished and die.

You’ve probably seen brown-headed cowbirds foraging on the ground in your yard or perched on the backs of cows and horses in fields. They are rather dull, with males sporting a chocolate-brown head and black glossy body, and the females being brown with a pale throat.

Seeing the birds on top of livestock, it’s easy to assume they’re plucking insects off the animals’ backs. Actually, though, they’re seeking insects stirred up from the ground as the livestock graze, said Saylor of WSU.

“They are proficient ground foragers,” he said.

Although the cowbird is named after its connection to cows, historically, the species evolved with bison. Centuries ago, millions of bison roved the Great Plains, and cowbirds followed the herds to eat kicked up insects and seeds, according to the Nature Conservancy. Well, this strategy posed a big problem: the birds had no time to build nests and start families because the bison were constantly roaming. So, the theory goes, they turned to other bird species to do it for them.

Then, in the 19th century, the mass slaughter of bison forced cowbirds to adapt once again by following cows, horses and other grazing livestock introduced by Euro-American settlers.

The cowbird has evolved to survive and prosper – much like humans, explained evolutionary biologist Clair Spottiswoode of the University of Cambridge, who has studied brood parasites for more than a decade.

“I can’t think of a topic that shows evolution more vividly,” she said in an interview with National Audubon Society magazine. “You cannot not be fascinated.”