There’s an arms race going on in this bird nest. Scientists are uncovering how each side fights back

Cowbirds are the quintessential deadbeat parents. They, and about 90 other bird species, abandon their eggs in other birds’ nests, leaving the burden of chick care to others. An arms race is the result: Cuckolded foster parents keep evolving ways to fight back, and deadbeats evolve countermeasures. Now, researchers have discovered how spots on an egg play a crucial role in a parent’s decision to keep an egg—or boot it from the nest.

One of the shiny cowbird’s (Molothrus bonariensis) most common victims is the chalk-browed mockingbird (Mimus saturninus). The mockingbird’s eggs are blue-green and spotted, whereas the cowbird’s eggs vary from pure white to brown and spotted. Researchers had assumed mockingbirds reject cowbird eggs that don't look like their own, in pattern and color. But the new study finds it’s not that simple.

To get a better sense of how mockingbirds decide which eggs to boot, evolutionary ecologist Daniel Hanley at Long Island University in Brookville, New York, and colleagues painted 70 3D-printed eggs a range of colors and put spots on half of them. They distributed these eggs among 85 mockingbird nests and checked several days later to see which eggs were still there.

Spots tended to make the mockingbirds hedge their bets and keep an egg, even if the color wasn’t “right,” Hanley and his colleagues report in the April issue of the Philosophical Transactions of the Royal Society B . For example, the mockingbirds removed unspotted brown eggs—a “wrong” color and pattern—90% of the time. But the birds were less sure when the egg had spots. They removed brown eggs with spots just 60% of the time, for example. In general, mockingbirds were more accepting of very blue eggs, even those that were much bluer than their own eggs. And when these blue eggs had spots, parents kept them more than 90% of the time.

“Adding spots can make an egg more acceptable,” says Sheena Cotter, an evolutionary ecologist at the University of Lincoln in the United Kingdom who was not involved with the work. So spots are an easy way for parasitic cowbirds to ensure their eggs are safe, even if they aren’t a perfect match.

But sometimes, the scofflaw bird has to do more than just make sure its eggs have spots. In Zambia, Mary Caswell Stoddard, an evolutionary biologist at Princeton University, and her colleagues recorded when 122 tawny-flanked prinias (Prinia subflava) rejected foreign eggs from their nests. The researchers noted the colors, sizes, and markings of each egg in each nest, and used a sophisticated pattern-recognition computer program to classify the shapes and orientations of the markings.

When the eggs are very similar to their own, the prinias use the shapes and positioning of the splotches to make the right call and keep an egg, she and her colleagues report in the same issue of the Philosophical Transactions of the Royal Society B . “The exact placement [of a spot] is very hard to mimic,” Cotter points out, making it possible for prinias to use that information when they are not sure whether an egg is theirs.

The two papers address the long-standing question of how parasitized birds recognize the difference between their own and imposter eggs, says Rose Thorogood, an evolutionary ecologist at the University of Helsinki who was not involved with the work.

These new studies show that sometimes the foster parents have become very smart—and persnickety—about what eggs they keep, Stoddard adds. After parasites evolve spots as a consistent part of the egg’s disguise, the foster parent evolves to use more brain power so it can remember more details about the spotting and hence become more discriminating. “What’s going on in the brains of [birds] is even more complex and interesting than we imagined,” she says.