How These Birds Can Spot Look-Alike ‘Imposter’ Eggs in Their Nests

Fork-tailed drongos can identify and reject egg forgeries, laid by African cuckoos, with nearly 94 percent accuracy, new research suggests

Black bird sitting on a piece of wood
Fork-tailed drongos have a signature color and pattern on their eggs, which helps them avoid getting duped by African cuckoos. Hans Stieglitz via Wikimedia Commons under CC BY-SA 3.0

Cuckoos have evolved a clever trick that allows them to reproduce without having to do the energy-intensive and time-consuming work of parenting: They lay their eggs in the nests of other bird species.

If the unsuspecting parents fail to notice the imposter egg, they’ll accidentally end up raising an interloper chick. And that happens at the expense of their own offspring: Once a cuckoo chick hatches, it will knock all other eggs out of the nest to get its adoptive parents’ full attention.

But one type of bird—the fork-tailed drongo—is particularly good at foiling the cuckoos’ sneaky plan known as brood parasitism, according to a new paper published Wednesday in the journal Proceedings of the Royal Society B.

Fork-tailed drongos—medium-sized, black or dark grey birds with red eyes that live throughout southern Africa—can suss out and reject African cuckoo eggs in their nests with 93.7 percent accuracy, the researchers found. The drongos likely evolved this special skill through natural selection, since it helps them avoid wasting their precious resources on another bird’s offspring.

Collage of brown speckled eggs
Can you spot the forgery? In all of these images, the right-most egg is the cuckoo egg—all the others are fork-tailed drongo eggs. Claire Spottiswoode

Scientists were able to figure this out by looking very closely at the eggs of both species: They studied the size, shape, color and pattern. On average, African cuckoo eggs and fork-tailed drongo eggs are nearly identical—they look so similar that even the researchers had a hard time telling them apart.

But the average didn’t tell the whole story. That’s because fork-tailed drongos produce an individualized, very specific egg pattern and color combination called a signature—and this varies greatly from bird to bird. African cuckoos can mimic these aesthetics almost perfectly, but they aren’t very accurate when it comes to matching the right pattern and color with the right drongo nest.

For example: One fork-tailed drongo may produce eggs that are covered in brown polka dots, while another may lay eggs with black speckles. Though an African cuckoo can mirror each of these patterns, they might lay a black-speckled egg in a nest full of brown polka-dotted eggs. This, in turn, makes it very easy for fork-tailed drongos to spot the forgery.

Tiny speckled egg in human hand
On average, African cuckoos are very good at matching the color and pattern of drongo eggs. Jess Lund

“Even though cuckoos have evolved excellent forgeries, individual cuckoos don’t target individual drongo nests that match their own eggs,” says study co-author Jess Lund, a zoologist at the University of Cambridge in England, in a statement. “This means that for each cuckoo egg laid, the likelihood that it will be a good enough match to that drongo’s signature is very low.”

This discovery highlights the importance of looking at “individual, case-by-case differences,” and not just population averages, as Lund tells the Guardian’s Nicola Davis.

Researchers unraveled this mystery over the course of four years in the Choma District of southern Zambia. To start, they analyzed the physical differences between cuckoo eggs and fork-tailed drongo eggs. Next, they ran a series of experiments in which they replaced an egg from each drongo’s nest with an egg from a different drongo, used as a proxy for African cuckoo eggs. Where possible, they matched the eggs to create “difficult decisions” for the drongos, per the paper.

Every day after that, they looked into the drongo nests to see whether the birds had accepted or rejected the imposter egg. From this, they were able to glean which characteristics of an egg predicted its rejection or acceptance.

By pairing these two kinds of data—the physical traits of cuckoo eggs and which traits lead to an egg’s rejection—the team devised a model that extrapolated the overall cuckoo egg rejection rate of 93.7 percent.

“It really highlights how effective evolution is at creating high rejection rates… because it’s so costly for drongos to raise a cuckoo themselves,” says Lund to New Scientist’s Christa Lesté-Lasserre.

An African cuckoo chick
An African cuckoo chick Claire Spottiswoode

The scientists were surprised by drongos’ egg-discerning skills, which suggest that a single, female cuckoo may only successfully produce two chicks throughout her entire lifetime. That doesn’t align with reality, however, as African cuckoos have strong numbers throughout Africa.

The team needs to conduct further research to understand their result, but one possible explanation is that fork-tailed drongos in the study area may be specialists among their species at identifying forgeries. Elsewhere, they could be fooled more often.

Either way, the study adds to the scant overall knowledge of defenses against mimicry and brood parasitism. At least for now, in this particular part of the world, fork-tailed drongos seem to have “the upper hand in their co-evolutionary arms race” against cuckoos, the researchers write in the paper.

Get the latest stories in your inbox every weekday.