How a parasitic bird with no parents learns what species it is

In P.D. Eastman’s classic picture book “Are You My Mother?” a baby bird hatches alone and goes on a quest. It asks a cow, a dog and even an excavator whether they might be its mother. Finally, the chick and its true mom reunite.

In nature, cowbirds also hatch without their parents present. Cowbird mothers leave their eggs behind in the nests of different species. Yet to grow up safely, the birds must join other cowbirds in flocks. They somehow need to learn what species they belong to.

Earlier research hinted that brown-headed cowbird chicks might reconnect with their parents. But a paper published recently in the journal Animal Behaviour found no evidence of a happy reunion. Instead, cowbird chicks learned who they were by hanging out with unrelated adult females.

Brown-headed cowbirds are what scientists call brood parasites. That means parents don’t raise their own young. Mothers sneakily lay eggs in other birds’ nests, and oblivious host parents rear the young brood parasite alongside their own offspring. (Chicks of the common cuckoo, another brood parasite, kill their foster siblings by shoving them over the side of the nest.)

Unlike most birds, a young brood parasite doesn’t get attached to its host parents. You can see this if you rear cowbirds by hand, said Mark Hauber, a comparative psychologist at CUNY Graduate Center in New York: “They start hating you at some point.” If a cowbird imprinted on a family of yellow warblers, say, and sought out warblers’ company as an adult, it would never find a mate and reproduce.

Beyond mating, “there’s tons of different benefits to knowing what kind of bird you are,” said Mac Chamberlain, an ornithologist at the University of Illinois Urbana-Champaign who works with Hauber. For example, cowbirds flock together, roost together and learn from each other on where to find food.

There’s safety in numbers, too. “They do like to hang out with their own species. Mostly because they themselves get beaten up by the other species,” Hauber said. Many birds will attack the brood parasites on sight.

In a 1995 study, researchers trapped juvenile and female adult cowbirds and sampled their blood. They found that some of the pairs captured together were likely to be related. The authors speculated that cowbird mothers might keep an eye on their developing chicks and reconnect after their offspring leave their hosts.

In the new study, Chamberlain expected to find something similar.

He spent three summers capturing cowbirds and doing genetic testing. Yet in his small sample, he found no evidence that those juveniles were hanging around their mothers or other relatives.

The result surprised him. “I was thinking that these adult cowbirds were kind of seeking out their juveniles, or at least hanging out in areas where they previously parasitized nests,” Chamberlain said.

But the pairings he found weren’t entirely random. Juveniles were much more likely to be with female adults than with males. This supported earlier work by Hauber showing that young cowbirds are attracted to the chattering calls of female cowbirds. Juveniles, all of which are born with female-like feathers, also seek out birds with plumage that matches their own.

“I would have been surprised if the juveniles had been found with their parents,” Fiorini said. To better rule out the possibility that young cowbirds meet up with their parents, she added, it would have been helpful to capture juveniles while they were still living near their host nests. However, the results fit with her research showing that young birds follow female chattering.

Cowbirds and their unconventional parenting tactics challenge our human assumptions about how animal families should look, Chamberlain said. His research supports the idea that the birds can reach adulthood without their biological parents (and won’t end up believing they’re a cow or a construction vehicle). They rely on unrelated female adults to show them what they need to know.