A lesser honeyguide emerging from its egg. (Claire Spottiswoode)

Some cuckoos are born assassins. Within a day or two of hatching, the infant birds—still blind, pink, and featherless—will start to evict the other residents of their nest, hurling them over the edge and to their death.

Technically, the evictions they carry out are from living quarters that aren’t even their own. The cuckoos are parasites, strategically placed by their mother into the abode of another species so they can mooch their way through adolescence. The more of their foster siblings they kill in cold blood, the more food and attention they can con out of their adoptive parents.

The acts are ruthless, but also remarkable physical feats. Fresh out of their shells, the birds are jacked, capable of hoisting hefty eggs or chicks—including some that weigh about as much as they do—onto their back before throwing them out like trash. “It’s like a newborn baby lifting a bowling ball,” Stephanie McClelland, a biologist at the Royal Holloway University of London, told me. “It’s just crazy.”

By peeping on cuckoo chicks during development, McClelland and her colleagues have homed in on one of the major strategies these birds, and several others like them, use to achieve their super-swole status at such a young age. In a new study, they describe how the animals exercise as embryos while they’re still incubating in the egg, a sort of prenatal CrossFit that preps them for the slaughterous rampage that follows. “It’s a home gym in the egg,” Mark Hauber, a biologist at the University of Illinois at Urbana-Champaign who collaborated with McClelland on the study, told me. By the time these birds hatch, their foster siblings already don’t stand a chance.

The notion that embryos can wriggle around in the womb or egg isn’t new. Human pregnancy is a prime example: Fetuses spend months executing a mix of twitches, stretches, and kicks that are thought to be an in utero trial run for the motions “they need to survive after birth,” Niamh Nowlan, a developmental-biomechanics expert at University College Dublin, told me. The situation isn’t all that different for birds, which start flexing their muscles just a few days after their egg is laid. These little full-body boot camps are vital: Animals that skip their pre-birth workouts tend to emerge with bones and muscles that are weak and underdeveloped.

More Stories

McClelland and her graduate adviser, Steven Portugal, decided to check if embryos could push that trend in the other direction—adding on exercise to the standard regimen of squirming, perhaps as a way to ensure that they’re born extra buff. If any animals were good candidates for pumping prenatal iron, they figured, cuckoos and other nest-invading birds, formally known as brood parasites, might be among them, given what they get up to in infancy.

Proving that, though, wouldn’t be easy. That’s because dozens of bird species are thought to engage in some form of brood parasitism, each with their own violent flair. Some, like the common cuckoo, are egg-tossing executioners; others, like the cowbirds that Hauber studies, let their host siblings survive, but still jostle them out of the way to beg, loudly and insistently, for food. Most macabre of all might be the lesser honeyguide, a fanged felon that will stab its nest-mates with the piercing-sharp hook that adorns the front of its beak or “shake them like a terrier” until they drop dead, Claire Spottiswoode, a biologist at the University of Cambridge and the University of Cape Town who studies honeyguides, told me. If McClelland wanted to find a connection between pre-hatch calisthenics and the shredded status of brood-parasite chicks, she’d need to spy on a whole lot of embryos while they were still in their shells.

A greater honeyguide nibbling on Stephanie McClelland’s finger. (Credit: Stephanie McClelland)

So McClelland enlisted Hauber, Spottiswoode, and several others to help her out. The researchers spent several years collecting the eggs of 14 bird species—some parasitic, some not—scattered across three continents. Using a device called an Egg Buddy, they beamed harmless lasers into their specimens, and tallied up how much the embryos were shifting around. The work was sometimes grueling, and not just because they were dealing with bloodthirsty birds: While gathering data in a rural region of Illinois, McClelland’s mobile laboratory, full of equipment and chemicals, was mistakenly flagged by locals as a meth lab.

But the project yielded exactly the results McClelland and her colleagues were hoping to see. While inside their egg, most brood parasites tended to fidget about more often than the host birds they tormented, especially during later stages of incubation. They were also jigglier than closely related birds that were raised by their own parents. Brood parasitism is thought to have arisen independently at least seven times in the avian family tree; “to see a similar pattern” across species and continents makes the team’s results especially compelling, Iliana Medina Guzman, a brood-parasite expert at the University of Melbourne who wasn’t involved in the study, told me.

The results aren’t totally ironclad. Nowlan, who wasn’t involved in the study, pointed out that the researchers weren’t able to check how the chicks in each species actually turned out, making it hard to confirm whether in-egg gains actually did produce brawnier birds. And the movement gaps among species also weren’t huge—more the difference between two casual weight lifters than a bodybuilder and a couch potato. Still, “when you look at a small difference in embryo movement or muscle development, it compounds on itself,” says Facundo Fernandez-Duque, an avian biologist who is advised by Hauber but wasn’t involved in the study. For a weary cuckoo chick, a few extra strength-training sessions might make all the difference between booting its fourth and final nest-mate and having to share its chow.

For Spottiswoode, the link feels intuitive, like confirmation of the years of work she’s done in the field, examining and handling murderous birds. Freshly hatched honeyguides even feel kind of toned. “They have an almost rubbery quality to them,” Spottiswoode said. That sinewy stuff is exactly what makes the baby birds’ bods so lethal. Even clutched in human hands, they’ll lunge and snap and fling their fangs about, “trying to find something, anything, to bite,” she said. Sometimes, survival of the fittest really does mean the fittest.