Newly hatched birds might seem like delicate, feeble creatures, but some chicks burst out of their eggshells ready to brawl. These little fighters exercise in their eggs before hatching to build up the strength needed to maim and murder their nestmates, a new study suggests.

The baby birds are part of a group of species known as brood parasites, whose moms lay their eggs in other birds' nests, leaving them for other bird parents to raise. Rather than expending the energy and time to rear their own offspring, brood parasite parents use both clever tricks and brute strength to give their babies a fighting chance with their foster family.

For example, the brown-headed cowbird (Molothrus ater) lays eggs that resemble those of its various hosts, while greater honeyguide (Indicator indicator) moms take the more aggressive approach of puncturing many of their hosts' eggs, thus killing the hosts' chicks and leaving their own offspring with less competition.

Although these bird moms set their kiddos up for success, once hatched, the chicks take matters into their own wings. Now, a new study, published Tuesday (Oct. 26) in the journal Proceedings of the Royal Society B: Biological Sciences, suggests that so-called embryonic movements — or egg exercise — may give the chicks a competitive edge over their foster siblings.

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Past studies in domestic birds, like chickens, have demonstrated that embryonic movements are key to growing chicks' development, said first author Stephanie McClelland, a doctoral candidate at the Royal Holloway, University of London in Egham. Studies suggest that hyperactivity in the egg results in a beefy, muscular chick, while other studies have shown that paralysis in an embryo causes stunted bone growth, malformed joints and reduced muscle tone.

So all birds benefit from embryonic movement, as it prepares them for the world beyond their eggshells. But the early lives of avid brood parasites tend to be uniquely strenuous.

Take, for example, the common cuckoo (Cuculus canorus). "Within just a day or two of being born, they lift an egg that's almost their own weight onto their back, and they shove it out [of] the nest," McClelland said. "Think of … a human baby trying to lift a bowling ball or something."

McClelland and her team wondered how newly hatched chicks pull off such feats of strength. They theorized that egg exercise might have something to do with it.

To test this idea, the team analyzed 437 eggs from 14 bird species, including five brood parasite species, their hosts, and several closely related, nonparasitic species, for comparison. McClelland plucked some birds' eggs from roosts on her university campus in the U.K., where domestic homing pigeons (Columba livia), a nonparasitic species, lay their eggs. But for the rest of the roughly 35 dozen eggs, she trekked to the U.S., the Czech Republic, Tanzania and Zambia, dropping in on field sites run by her collaborators.

To monitor each chick's movement within its egg, the researchers used a device called an Egg Buddy, which shines a beam of infrared light through the egg and records when that beam gets disrupted — for instance, when the bird's muscles twitch. For each egg, the team calculated the "embryo movements per minute" at five time points within its incubation period, to see how the growing chicks' rate of movement changed through time. The team standardized these time points across species so they would match specific stages of embryonic development.

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In general, avian brood parasites have very short incubation periods; for example, the brown-headed cowbird's entire incubation period lasts about 10 days, according to McClelland. "These birds have a really short amount of time in the egg to go from really a bunch of goo into a real bird," she said. This is thought to be another survival strategy, in that short incubation periods give the host bird less time to spot and reject the eggs. Plus, by hatching early, the more murderous parasites get a chance to destroy the host birds' eggs, or kill off the host chicks as soon as they hatch. And gentler species, like the brown-headed cowbirds, use their muscles to perform "exaggerated begging" behaviors for their foster parents, monopolizing the food supply before their nestmates hatch.

Overall, in their brief incubation periods, the parasitic species showed higher rates of embryonic movement than did the hosts and nonparasitic species, the team found. And in general, the parasites' movement increased at a steeper rate over the course of their incubation periods, compared with the other birds. This was especially evident in the later embryonic stages, just before hatching; the common cuckoos, brown-headed cowbirds and lesser honeyguides (Indicator minor) wriggled with particular gusto at this stage of development.

This trend held across the parasitic species. But overall, there was some variation in the movement rates between species. For example, the lesser honeyguides showed a far higher rate of movement than the greater honeyguides, even though both are parasitic species that murder their foster siblings.

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Perhaps the difference is due to the early lives of these two honeyguides, which differ drastically: Not only do greater honeyguide moms poke holes in the host chicks' eggs, leaving their parasitic babies with less competition, and on top of that, the honeyguide chicks are larger than their hosts, little bee-eaters (Merops pusillus). Meanwhile, lesser honeyguides slip their eggs into the nests of black-collared barbets (Lybius torquatus), a larger bird species. Once hatched, the lesser honeyguides massacre the hefty host chicks by snagging them with a beak hook and shaking them vigorously.

"So birds have quite a hard task when they hatch in a nest full of big, chunky barbet nestmates that they then have to bite and shake with their hook," McClelland said. This may explain why the lesser honeyguides do more egg exercise before hatching than the greater honeyguides, which have no trouble shaking their puny nestmates to death. And in fact, greater honeyguides' embryonic movement is very similar to that of their hosts, suggesting that the birds don't need much muscular training to take out the little bee-eaters.

Looking forward, McClelland said she plans to study what factors actually trigger embryonic movement in the various bird species. Perhaps environmental factors, like temperature, play a role in when and how vigorously the developing chicks move, she said. But because the parasites and hosts share a nest, McClelland suspects that hormonal and genetic factors may be the primary drivers behind egg exercise. The team is also looking into birds' energy usage during this critical period of development, given that they're limited to using the yolk within their eggs.

"It's quite a mystery what's going on inside these eggs," McClelland said. The new study is "really a starting step … There will definitely be a lot more work [that] needs to be done to really connect how this movement is shaping these birds."

Originally published on Live Science.