These bird moms lay their eggs in other species' nest, and never come back for them

CBC

06-07-2025

When a female cowbird is ready to have a baby, she'll stake out another bird's nest, wait until the coast is clear, then slip in, stealthily lay her egg and leave it behind for another family to raise.
Most other baby songbirds in that situation would imprint on their adoptive mothers and grow up believing they were members of another species. Cowbirds, however, have no such identity crises.
"It's very clear that at a certain age they get very tired of their host parents and they want to get out of town," Mac Chamberlain, an ornithologist at the University of Illinois Urbana-Champaign, told As It Happens.
Previous research suggested that once the chicks were reared, their biological mothers would come back for them and teach them how to be cowbirds.
But a new study, published in the journal Animal Behaviour, found that's not the case. While they do eventually find other members of their own species, they get along just fine without mom.
Brood parasites
Cowbirds are what's known as brood parasites, meaning they allow other species to rear their young, often to the host nest's detriment.
Some birds are clever enough to recognize an unfamiliar egg in their midst and get rid of it. But once the cowbird hatches and starts squawking for food, most mothers will instinctively feed them, none the wiser of the deception.
Less than one per cent of bird species worldwide use this mating strategy, Chamberlain said.
"It's very cool and it's very novel, I think, compared to other birds," he said.
Some brood parasites, like cuckoo birds, go on a killing spree the minute they hatch, pushing their host siblings out of the nest to their untimely deaths so they don't have to compete for resources.
"Cowbirds are unique in the fact that they're nest sharers," Chamberlain said. "The only harm they really do to their nest mates is just being an extra mouth to feed."
While not as brutal as cuckoos, it's still not great for the host family.
"Cowbirds hatch earlier and start begging, many have argued, more frequently and more intensely [than their nest mates], and so they end up getting more food," Matthew Louder, a former scientist who studied brood parasites at Texas A&M University, told CBC.
"If you cannot bring enough food to feed your other ones, you might lose some of the other offspring."
If they reunite with mom, it's probably random chance
Within about a month, the little imposters leave the nest for good.
From there, Chamberlain says they have a short window to find an adult cowbird and learn the ropes before grouping together to migrate south.
It's not clear what happens if they don't find an adult in time, but Chamberlain says cuckoos who fail in this quest never learn their species-specific behaviours and local dialect, and end up ostracized from their flock.
In a 1995 study, researchers found and trapped pairs of adult and juvenile brown-headed cowbirds, sampled their blood, and discovered many of these duos were related.
The authors theorized the juvenile cowbirds were reuniting with their mothers before flying south. The mothers, they suggested, may even keep watch over their hatchlings from afar, then lead them out of the nest when it's time.
But this latest study was not able to reproduce those findings. Researchers spent three years capturing and genetically testing brown-headed cowbirds — 122 adults and seven juveniles altogether — and found the young birds were more likely to be found with unrelated female adults.
Scientists believe the juveniles find and latch onto adult females, in part, because they have similar looking plumage, but also because they're instinctively drawn to their chattering calls.
Louder says he was not surprised by the findings. In his own research, he says he'd already shown that juvenile cowbirds leave their nests all on their own.
But Chamberlain says this doesn't mean the 1995 study was flawed. This new research, he says, was conducted in Illinois, where cowbirds are more populous than on the east coast, where the previous study was carried out.
"If you're .... in an area where there's only really one breeding female cowbird, if you bump into a female cowbird, it's likely going to be your mom," he said.
Stephen Rothstein, a professor emeritus at the University of California Santa Barbara who wasn't involved in the study, says while the methodology is sound, the sample size of juveniles is too small to draw any hard conclusions.
Nevertheless, he said in an email that he agrees juvenile cowbirds are "unlikely to have a mechanism by which they can identify their biological parent from another adult cowbird."
But since cowbirds have defined territories, he says it's possible they'll meet their parents again, just by random chance.
While some might find this breeding strategy cold, the people who study cowbirds defend them passionately.
They're a native species doing what they're meant to do, says Louder, and they're no more cruel than hawks and other birds of prey who snatch up other birds or eggs to eat.
Except in rare circumstances where cowbirds heavily outnumber an endangered species, he says, they don't pose population-level threats to other birds.