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

Sheena Goodyear · CBC Radio

· Posted: Jul 04, 2025 6:38 PM EDT | Last Updated: July 4

When a pistillate cowbird is acceptable to person a baby, she'll involvement retired different bird's nest, hold until the seashore is clear, past gaffe in, stealthily laic her egg and permission it down for different household to raise.

Most different babe songbirds successful that concern would imprint connected their adoptive mothers and turn up believing they were members of different species. Cowbirds, however, person nary specified individuality crises.

"It's precise wide that astatine a definite property they get precise bushed of their big parents and they privation to get retired of town," Mac Chamberlain, an ornithologist astatine the University of Illinois Urbana-Champaign, told As It Happens.

Previous probe suggested that erstwhile the chicks were reared, their biologic mothers would travel backmost for them and thatch them however to beryllium cowbirds.

But a caller study, published successful the diary Animal Behaviour, recovered that's not the case. While they bash yet find different members of their ain species, they get on conscionable good without mom. 

Brood parasites

Cowbirds are what's known arsenic brood parasites, meaning they let different taxon to rear their young, often to the big nest's detriment. 

Some birds are clever capable to admit an unfamiliar ovum successful their midst and get escaped of it. But erstwhile the cowbird hatches and starts squawking for food, astir mothers volition instinctively provender them, nary the wiser of the deception.

Less than 1 per cent of vertebrate taxon worldwide usage this mating strategy, Chamberlain said.

"It's precise chill and it's precise novel, I think, compared to different birds," helium said. 

Some brood parasites, similar cuckoo birds, spell connected a sidesplitting spree the infinitesimal they hatch, pushing their big siblings retired of the nest to their untimely deaths truthful they don't person to vie for resources. 

"Cowbirds are unsocial successful the information that they're nest sharers," Chamberlain said. "The lone harm they truly bash to their nest mates is conscionable being an other rima to feed."

While not arsenic brutal arsenic cuckoos, it's inactive not large for the big family.

"Cowbirds hatch earlier and commencement begging, galore person argued, much often and much intensely [than their nest mates], and truthful they extremity up getting much food," Matthew Louder, a erstwhile idiosyncratic who studied brood parasites astatine Texas A&M University, told CBC.

"If you cannot bring capable nutrient to provender your different ones, you mightiness suffer immoderate of the different offspring."

If they reunite with mom, it's astir apt random chance

Within astir a month, the small imposters permission the nest for good. 

From there, Chamberlain says they person a abbreviated model to find an big cowbird and larn the ropes earlier grouping unneurotic to migrate south.

It's not wide what happens if they don't find an big successful time, but Chamberlain says cuckoos who neglect successful this quest ne'er larn their species-specific behaviours and section dialect, and extremity up ostracized from their flock.

In a 1995 study, researchers recovered and trapped pairs of big and juvenile brown-headed cowbirds, sampled their blood, and discovered galore of these duos were related.

The authors theorized the juvenile cowbirds were reuniting with their mothers earlier flying south. The mothers, they suggested, whitethorn adjacent support ticker implicit their hatchlings from afar, past pb them retired of the nest erstwhile it's time. 

But this latest survey was not capable to reproduce those findings. Researchers spent 3 years capturing and genetically investigating brown-headed cowbirds — 122 adults and 7 juveniles altogether — and recovered the young birds were much apt to beryllium recovered with unrelated pistillate adults.

Scientists judge the juveniles find and latch onto big females, successful part, due to the fact that they person similar looking plumage, but besides due to the fact that they're instinctively drawn to their chattering calls. 

Louder says helium was not amazed by the findings. In his ain research, helium says he'd already shown that juvenile cowbirds permission their nests each connected their own.

But Chamberlain says this doesn't mean the 1995 survey was flawed. This caller research, helium says, was conducted successful Illinois, wherever cowbirds are much populous than connected the eastbound coast, wherever the erstwhile survey was carried out.

"If you're .... in an country wherever there's lone truly 1 breeding pistillate cowbird, if you bump into a pistillate cowbird, it's apt going to beryllium your mom," helium said.

Stephen Rothstein, a prof emeritus astatine the University of California Santa Barbara who wasn't progressive successful the study, says portion the methodology is sound, the illustration size of juveniles is excessively tiny to gully immoderate hard conclusions.

Nevertheless, helium said successful an email that helium agrees juvenile cowbirds are "unlikely to person a mechanics by which they tin place their biologic genitor from different big cowbird."

But since cowbirds person defined territories, helium says it's imaginable they'll conscionable their parents again, conscionable by random chance.

While immoderate mightiness find this breeding strategy cold, the radical who survey cowbirds support them passionately.

They're a autochthonal taxon doing what they're meant to do, says Louder, and they're no much cruel than hawks and different birds of prey who snatch up different birds oregon eggs to eat.

Except successful uncommon circumstances wherever cowbirds heavy outnumber an endangered species, helium says, they don't airs population-level threats to different birds. 

"It's a large happening successful the birding community. Everyone hates cowbirds," Louder said. "I'm ever trying to benignant of archer everyone that they're truly cool. It's hard to beryllium a cowbird."