Greg Schechter via Wikimedia Commons

Australia’s little bronze cuckoos (Chalcites minutillus) lay odd looking eggs. They have a thick coat of pigment in the outermost layer of the shell that gives them a dark olive or brown color and reflects very little light. Even weirder is that they don’t look a thing like the white speckled eggs of the large-billed gerygone (Gerygone magnirostris).

That there’s little resemblance between one bird’s egg and another’s makes sense for most species, but not these two. Bronze cuckoos, like many of their cuckoo cousins, are brood parasites that outsource their parental duties by ditching their eggs in other birds’ nests and letting them raise the chicks. Laying eggs that resemble and blend in with the hosts’ makes the cuckoo’s trickery easier, and many species have evolved eggs that closely mimic those of their preferred host species. To defend against parasitism and avoid getting saddled with some other bird’s chicks, many hosts, in turn, have developed a keen sense for spotting the differences between the lookalike eggs and their own so that they can remove the impostors. That puts pressure back on the cuckoos to lay even better egg mimics. This back-and-forth of adaptation and counter-adaptation between two organisms is an example of what biologists call a coevolutionary arms race, one in which the cuckoo and its poorly-disguised brown eggs appear to have been left behind.

But gerygones rarely reject cuckoo eggs from their nest, and Ros Gloag, a biologist at the University of Sydney, doesn’t think that the cuckoos are losing the arms race, just going about fooling their hosts in a different way. Instead of mimicking the gerygone’s eggs, the cuckoos might be hiding theirs in plain sight. Gerygones build domed nests that are very dark inside, and the cuckoo eggs’ dark pigment makes them almost indistinguishable—to the eyes of a birdfrom the nest lining. If the gerygones can’t see the cuckoo eggs, then they can’t remove them.

If the strategy works, that’s all well and good for the cuckoos, but Gloag still wondered why they would hide their eggs when so many closely related species, including some other types of bronze cuckoo, rely on egg mimicry instead. Now, with a new study, she thinks she’s found the reason: other cuckoos.  

Gerygone nests are often hosts to more than one cuckoo, which complicates the arms race being fought. Before a little bronze cuckoo lays her egg in another bird’s nest, she removes one of the existing eggs. And once the cuckoo eggs hatch, the nestlings usually push all other eggs or chicks out of the nest so they receive their foster parents’ full attention. If two cuckoos target the same nest one after the other, cuckoo #1’s egg is at risk of being removed by cuckoo #2 when she shows up. And if cuckoo #2 doesn’t get rid of that egg, her baby is in danger of getting thrown out by cuckoo #1’s once it hatches. 

In a competition like this, a cuckoo can give her offspring a leg up by picking out and removing any cuckoo egg already in the nest, which should drive egg adaptations that reduce the risk of removal—like eggs that are hard to see. 

If a mother cuckoo has to fight an arms race on two fronts, and fool or evade detection by both host birds and her cuckoo competition, then the dark eggs start to make more sense. “In this case, there is a clear benefit of crypsis over mimicry,” Gloag and her team say in their paper. “Because the risk of removal of a mimetic egg would be equal to that of other eggs in the nest, whereas a cryptic egg would have a risk of removal lower than chance.”

To untangle just what’s going on with these eggs, Gloag and her colleagues took some finch eggs and painted some of them dark olive and the rest of them bright white. They then planted one of each egg in gerygone nests in northeastern Australia and checked the nests periodically over a few days, looking to see if their eggs had been rejected by either the gerygones or a cuckoo. If the bronze cuckoo eggs’ dark color kept them hidden from the other birds, Gloag expected that their similarly dark olive-painted eggs would be removed less often than their highly visible white ones.

That prediction didn’t pan out with the gerygones, who hardly rejected any out-of-place eggs. While they left natural cuckoo eggs alone when their nests were parasitized, the gerygones did remove at least one of each type of painted egg. If dark eggs don’t go unnoticed by gerygones, it’s unlikely that they’d be the ones driving the cryptic egg color. (Why don’t the natural cuckoo eggs get rejected, then, if they’re not hidden from the gerygones? The researchers think that if the birds can spot the foreign eggs, they still might not be able to remove them because they’re too large to move). 

Meanwhile, the cuckoos coming to the nests almost always removed an egg before laying their own, but tended to leave the dark ones alone and tossed the olive-painted eggs only around 10 percent of the time. 

The results suggest that the cuckoos are hiding their offspring from each other, not from their hosts, and that cryptic eggs are the result of an arms race being fought within one group of animals, not between two different ones.  

How might this arms race escalate? The competition should drive adaptations for better egg crypsis and better egg detection, so it’s possible that the cuckoos could keep one-upping each other when it comes to finding each others’ eggs and hiding their own. But Gloag thinks that the cuckoos can only get so good at picking out other cuckoo eggs because they’re crunched for time. The researchers found that when a little bronze cuckoo mom visits a nest, she spends 15 seconds or less there. “During her short visit, she must both select an egg for removal and lay an egg, all while clinging awkwardly half-out of the nest and enduring the attack of gerygone hosts,” they write. “Thus, the issue of timing, combined with the limitations of avian vision in low light, may prevent the evolution of more sophisticated egg discrimination,” and curb the cuckoo-vs-cuckoo arms race.