Most birds build their own nests and incubate their own eggs. However, some birds like the cuckoo have managed to get around this inconvenience by simply laying their eggs in the nests of other species and letting someone else do the hard work of keeping the eggs warm and protected until the chick hatches. The ‘host’ (the poor sucker who ends up taking care of the other birds’ eggs) does everything they can to try and make sure the eggs that they’re sitting on are just their own. On the other hand, the ‘brood parasite’ (the freeloader bird that just lays in the nests of others) does everything they can to make their eggs indiscernible from the eggs of their host.

Of course, neither host nor parasite is trying to achieve anything consciously; their eggs are shaped by evolution and there is natural selection both on the brood parasite to produce eggs that are very similar to the host they are attempting to parasitize, and on the host to be extra discerning in telling which eggs are their own.

On a side note, something that seems quite amazing from our human perspective is that while there seems to be a lot of selection on hosts to detect imposter eggs, there is little selection on their ability to discern whether their chicks in their nest are their own. Once the eggs have hatched the likelihood of them belonging to the host bird is high enough that it’s not worth risking chucking out any odd-looking chicks. That’s how this reed warbler ends up feeding this cuckoo chick as if it were its own (even though it is twice the size of its own chicks).

How birds detect whether eggs in their nest might be another birds’ ‘in disguise’ is done in a number of ways. It’s long been known that host birds will pay attention to cues like the colour, shape or size of their eggs when working out whether they might have an imposter egg nestled in with their own. However, a new study carried out in New Zealand has found that birds might use yet another, more unusual method to detect a foreign egg. In the species they looked at, blackbirds and thrushes, it seems that individuals might actually pay attention to how their eggs are arranged in order to better spot an imposter.


This reminds me of some more obsessive-compulsive humans that I’ve come across who leave their possessions in a particular way to know whether someone has borrowed a pen. A friend even once told me about a flatmate they had who would draw a line in felt-tip on their milk carton each time they used it in order to detect if anyone else stole some in their absence.






Something similar seems to be going on in blackbirds and song thrushes. To investigate it, the researchers added clay model eggs to the nests of these birds to see which individuals would chuck the fake egg out of their nest. The fake eggs were made to be similar to the eggs of cuckoos that naturally lay eggs in the nests of these birds. What the scientists found was that the birds that were more consistent in how they arranged their eggs (what I like to think of as the more uptight birds) were also better at rejecting imposter eggs. Similarly, those birds that arranged the eggs in their nest with no apparent consistency or order (a bit like my desk) were worse at rejecting eggs.

This would seem to imply that birds that are very organised, keeping their eggs in a particular arrangement in their nest, are thus better at spotting when there is a new egg messing up their carefully-constructed system. And this indeed could be the case. However, following the mantra of scientists: correlation is not causation, and the finding that birds that had more organised nests also were better at rejecting the imposter eggs could be due to something else completely (to pick a random example, maybe birds with better eyesight have more organised nests and are better at rejecting fake eggs). In order to really be sure whether nest-organisation and egg-rejection are causally related, scientists need to carry out an experimental study where they alter how organised birds’ nests are to see whether this affects their ability to reject imposter eggs.


Thanks to Julie Hecht for drawing my attention towards this interesting article






Polačikovà, L. et al. Egg arrangement in avian clutches covaries with the rejection of foreign eggs. Animal cognition. DOI 10.1007/s10071-013-0615-1

Photo credits

Eastern Phoebe (Sayornis phoebe) nest with one Brown-headed Cowbird (Molothrus ater) egg: GalaWebDesign

Reed Warbler feeding a cuckoo chick: Per Harald Olsen

Blackbird and song thrush: Andreas Trepte

M&Ms: tori.malea