Oriental and Common Cuckoos are a nightmare for nesting songbirds. Instead of raising their own chicks, these brood parasites sneak their eggs into other species’ nests. When the cuckoo eggs hatch—and they’re adapted to hatch first—the newborns use their first breaths of life to shove the native eggs out of the nest, leaving more space and food for themselves. It’s one part clever, one part insidious.
For years, these cuckoos have kept to Eurasia, where native birds have learned to defend themselves against the brood parasites over thousands of years. But now, some scientists are concerned that they might soon colonize Alaska, where local songbirds are utterly clueless to the parasites’ deceptive ways, according to a new study in the Journal of Field Ornithology.
“North American birds have absolutely no defense,” says Vladimir Dinets, a biologist at Okinawa Institute of Science and Technology and the study’s lead author, who’s been researching this behavior for years. “If the cuckoos start breeding in Alaska, they’ll have a huge impact on these birds.”
It comes down to host adaptations, Dinets says—or lack thereof.
In the sparse willow forests of Siberia, birds like the White Wagtail and Red-throated Pipit which host cuckoo eggs have learned to cope with the avian parasites. They can recognize an alien egg, and when they do, they either kick it out of the nest before it hatches or simply abandon the nest altogether to start anew.
But in North America, birds have never encountered parasitic cuckoos—or any species that lays chicks that deliberately murder their nestmates. (Brown-headed cowbirds, North America’s most common nest parasites, don’t outright kill the other chicks in their hijacked nest; they try to outcompete them for resources instead.) And so for millennia, Alaskan birds have had no need to detect alien intruders.
The study's authors, however, believe that could soon change. Cuckoos populations, and especially that of the Common Cuckoo, are expanding on the Siberian side. Driven by a shifting climate, Common and Oriental Cuckoos are now breeding close to the Bering Strait, the authors write, and stray birds can be spotted in the eastern reaches of Alaska during the breeding season—a sign that they are moving in.
Other scientists say that these sightings are vagrant birds or mere anamolies and not cause for concern. They have been seen in Alaska for decades, after all, and no such invasion has taken place. Nor is there strong evidence that they are breeding—yet.
“The question I was trying to answer was what would happen if they started to breed,” Dinets says of his newest study.
Breeding cuckoos, he thought, could be a disaster for local birds. And so, last summer, he put his question to the test. Working with a small team of researchers, Dinets compared anti-parasite defense mechanisms between birds in Siberia and those in Alaska. They 3D-printed more than 100 cuckoo eggs (as one does), which they placed among host nests on either side of the Bering Strait. Then, they waited and watched. Would the hosts accept the fake cuckoo eggs as their own, kick them out, or abandon their nest altogether?
Their results revealed a striking contrast. Host birds in Siberia, where cuckoos are common, rejected almost two thirds of the fake eggs. But in Alaska, the potential host birds rejected just a single egg of the 96 that were deployed.
“We found striking differences between the two locations,” says Mark Hauber, an author on the study and ornithologist at the University of Illinois who runs a lab focused on brood parasitism. “We think that these birds are not prepared for any type of egg rejection." (The only bird to reject a cuckoo egg was a Red-throated Pipit, which is likely descended from a Eurasian lineage that recently colonized the state.)
Despite the bleak conclusion of this study, the odds that a full-on invasion is likely or even underway are low, say other experts.
"The cuckoos are not moving West to East," says Dan Gibson, a research affiliate at the University of Alaska's Museum of the North, who has spotted a few cuckoos in the state himself—including a courting pair. "They’re getting carried by the movement of weather systems." And the reason there have been more sightings in the past 50 years? "More people are paying attention in far-flung regions of Alaska," he says, where these birds are occasionally found.
Gibson isn't the only one to raise an eyebrow at the idea of an invasion. Kenn Kaufman, a renowned bird expert and field editor at Audubon, explains that cuckoos found in Alaska are often "on their last legs" because they can't find food in the foreign habitat. Further, he says, the uptick in sightings occurred in the 1970s and 1980s—and they don't appear to be increasing since then.
"They lay out solid evidence that cuckoos could do major damage if they were to invade Alaska in large numbers," he wrote in an email. "But so far I haven’t seen any evidence that such an invasion is likely any time soon."