Parasitic Chicks Exercise In Egg To Get Buff And Destroy Nestmates


Rachael Funnell


Rachael Funnell

Writer & Senior Digital Producer

Rachael is a writer and digital content producer at IFLScience with a Zoology degree from the University of Southampton, UK, and a nose for novelty animal stories.

Writer & Senior Digital Producer

brood parasite embryo exercise

After killing their nestmates, brood parasites can grow to be bigger than their "adopted" parents. Image credit: Francesco de Marco /

The early bird gets the worm, and new research suggests that chicks that get their reps in before they’ve even hatched get a head start too. The claim pertains specifically to brood parasite chicks that are sneakily laid in nests belonging to other birds. When the chick hatches, it will benefit from the parental care of a bird who doesn’t realize the chick isn’t their own. These invaders sometimes even go as far as hoisting the other eggs out of the nest so they can reap all the rewards alone.

Now, a new study published in the journal Proceedings of the Royal Society B has found that embryonic movement is greater in parasitic chicks compared to those that “belong” in the nest. The seemingly premature exercise likely benefits the birds by strengthening their musculoskeletal system so that by the time they bust out of the shell they’re already ready to start kicking ass and taking names in the form of squishing and evicting their nestmates.


Considering their impressive physical feats in early life, the researchers hypothesized that embryonic movement could be how these birds come into the world so buff. To find out, they monitored the movement of embryos in a range of birds, both parasitic and non-parasitic. Sure enough, using a portable digital egg monitor called Egg Buddy, they observed that brood parasitic birds were getting in a lot more exercise while still in the egg, moving considerably more than the non-parasitic birds.

“Using a phylogenetically controlled analysis, we found that brood parasites exhibited significantly increased muscular movement during incubation compared to non-parasites,” wrote the study authors. “This suggests that increased embryo movement may facilitate the development of the stronger musculoskeletal system required for the demanding tasks undertaken by young brood parasites.”

To the human eye, it can be quite obvious when a chick doesn't belong. Image credit: Per Harald Olsen CC BY-SA 3.0, via Wikimedia commons

The eggshells of brood parasite species are also known to be thicker compared to non-parasitic birds, meaning these chicks need to be tough if they’re to break out of the egg at all. Once out, however, the trials continue – brood parasite chicks will kill and evict their unwilling “siblings”, with some birds even hatching with curved backs, perfect for scooping eggs out of the nest (see photo below).

The researchers found that the methods of the parasitic chicks also correlated with different degrees of embryonic movement. Greater honeyguides parasitize the nests of birds whose chicks are smaller than their own, and these parasitic chicks kill their nestmates by pulverizing the eggs before anyone else has had a chance to hatch. Lesser honeyguides, on the other hand, parasitize nests that have bigger chicks than their own, and the parasitic chick takes on hatchlings rather than eggs.

Cuckoo chicks use their backs to push other eggs out of the nest. Image credit: Vishnevskiy Vasily /

This could explain why lesser honeyguides spend a lot of time exercising in the egg. They have one hell of a task ahead of them once they’re out in the world, taking down an entire brood of chicks the same size or bigger than them. Conversely, greater honeyguides proved to move less even than their host chick – an observation that makes sense in the context that all they need to do once they’re out of the egg is blend up a defenseless omelet.

While the researchers express that further research is needed to identify which specific aspects of a parasitic chick’s lifestyle determine its embryonic movement, the research goes to show that the politics of nesting birds is as rife with impressive adaptations as it is scandal.

“Here, we have shown that the behaviour of the embryo during development could shape the physiology of brood parasites, and so may be a key factor in the successful exploitation of their hosts,” concluded the study authors. “The behaviour of brood-parasitic hatchlings is extraordinary and demonstrates their exceptional physical abilities.”

[H/T: Live Science]


  • tag
  • animals,

  • birds,

  • parasites,

  • eggs,

  • chicks