Cowbirds are infamous for being absentee parents, but that doesn't necessarily mean their chicks will have a difficult childhood. Like cuckoos, cowbirds are brood parasites, which means they leave their eggs in the nests of other species, dodging the duties of parenthood by tricking other birds into raising their babies for them.

This can lead to a heartbreaking scenario for those unwitting foster parents, who spend time and energy raising a chick that not only isn't theirs, but whose success often comes at the expense of their actual offspring.

And so bird species targeted by brood parasites have evolved some tactics to help them avoid this con, such as paying closer attention to the eggs in their nests and using more brain power to identify any eggs that seem unfamiliar. Cowbirds and other brood parasites, however, have evolved countermeasures to prevent their eggs from being outed, namely by producing variable eggshells that invite less scrutiny.

chalk-browed mockingbird and shiny cowbird The chalk-browed mockingbird (left) and shiny cowbird (right) are both common and relatively widespread across much of South America. (Photos: Joel Santana/Shutterstock and Helissa Grundemann/Shutterstock)

This has grown into a co-evolutionary arms race, as the hosts' egg-recognition skills puts selective pressure on brood parasites to lay less conspicuous eggs, which in turns puts more pressure on the hosts to improve their egg-recognition skills.

A new study takes a closer look at this phenomenon, focusing on the relationship between two common South American birds: the shiny cowbird (Molothrus bonariensis) and one of its favorite victims, the chalk-browed mockingbird (Mimus saturninus). Published in the Philosophical Transactions of the Royal Society B, the study reveals how mockingbirds use the colors and patterns of eggs in their nests to help them decide which ones to keep and which to throw out.

This is a fraught decision: The mockingbirds obviously don't want cowbird eggs in their nest, but they also don't want to be so zealous in evicting cowbirds that they accidentally kick out their own eggs. It may seem obvious that mockingbirds would just reject any eggs that don't match the color and pattern of their own eggs, but the new study suggests it's a little more complicated than that.

Don't have a cowbird

chalk-browed mockingbird with eggs in nest These video stills show a chalk-browed mockingbird rejecting a foreign egg from her nest. (Photos: Analía V. Lopez)

To test how mockingbirds make this decision, a team of researchers from the U.S., Argentina and the Czech Republic placed a variety of fake eggs in mockingbird nests across Reserva El Destino, a 500-hectare (1,235-acre) wildlife reserve near the town of Magdalena in Buenos Aires Province, Argentina. The eggs were 3D-printed models, based on the actual mass and dimensions of shiny cowbird eggs found at this site.

The researchers hand-painted two sets of eggs along a gradient of blue-green to brown, using a previously published method to match "the natural gradient of avian eggshells." They also painted spots on one set of eggs, applying a pattern modeled after a shiny cowbird egg chosen randomly from the local population.

These eggs were then taken to Reserva El Destino, where the researchers found 85 mockingbird nests, adding one randomly selected fake egg to each. They monitored all the nests for five days, and after excluding 15 that were either attacked by predators or abandoned, ended up with a final sample size of 70 nests. Any eggs that were still in a nest after five days were considered accepted, the researchers note, while any that went missing during this period were considered rejected.

The video below, filmed by co-author and University of Buenos Aires ecologist Analía V. López, shows two of the mockingbirds' reactions to unspotted vs. spotted eggs:

Spots had an interesting effect on mockingbird parents, often prompting them to play it safe and keep an egg even if the color wasn't right. Most mockingbirds were not fooled by unspotted brown eggs, which stand apart in both color and pattern, and those eggs had a rejection rate of more than 80 percent. But spots seemed to inspire some hesitation, presumably leading the parents to worry about discarding one of their own eggs. The rejection rate for brown eggs with spots, for instance, was only about 60 percent. The mockingbirds showed a bias for blue eggs, even accepting some with a bluer hue than their own eggs. And when blue eggs also had spots, the rejection rate dropped below 10 percent.

"Mockingbirds have spotted eggs, therefore it makes sense that they should be more willing to accept a spotted egg," explains lead author Daniel Hanley, an evolutionary ecologist at Long Island University Post, in an email to MNN. "Through a unique experimental design, we were able to measure how much spots contributed to a mockingbirds’ decision to tolerate a foreign egg."

The study suggests mockingbirds still may care more about egg color than spots, Hanley says, but both factors are important. The birds showed a clear bias for bluer eggs over browner ones, but when their discriminatory efforts grew more difficult — which Hanley and his colleagues achieved by adding spots, thus reducing the differences between "right" and "wrong" eggs — rejection was less likely.

The mockingbirds do sometimes seem conflicted about whether to keep or reject an egg, Hanley says, although it depends on the female and the context. "Some birds seem to know immediately, while others take a little more time," he says.

Brood awakening

flock of shiny cowbirds Shiny cowbirds are highly social, often forming flocks like this one in Brazil. (Photo: Pamela C.P. Olczyk/Shutterstock)

The new study is part of a theme issue for Philosophical Transactions of the Royal Society B, dedicated to "the co-evolutionary biology of brood parasitism." It looks at a wide range of brood parasites, including birds as well as lesser-known examples like the cuckoo catfish or brood-parasitic bees and butterflies. Because brood parasites rely on other species to raise their offspring, and because those other species could lose their own offspring if they don't spot the ruse, these creatures provide "an illuminating system for researching co-evolution," the issue's editors write.

Some victims seem savvier about thwarting brood parasites than others, likely due to variations in the parasites' mimicry abilities and the threats they pose to their hosts. In another study from this issue, for example, Princeton University evolutionary ecologist Mary Caswell Stoddard and her colleagues note that cuckoo finches can closely mimic the eggs of tawny-flanked prinias. In response, the prinias have evolved to use "higher-level pattern attributes" to identify foreign eggs, including details about the shape and the orientation of markings on the eggshell.

eggs of cuckoo finches and their hosts Examples of cuckoo finch eggs (two columns at right) alongside eggs of their hosts, the tawny-flanked prinia and the red-faced cisticola. (Photos: Claire N. Spottiswoode, Martin Stevens/Proceedings of the Royal Society B)

For chalk-browed mockingbirds, brood parasites may not have forced the same level of scrutiny, but there's still time. Given the apparent success of shiny cowbirds, it seems likely this co-evolutionary arms race is far from finished.

"Our findings suggest that this host has not yet adapted the ability to discriminate fine-grained differences in eggshell patterns, but instead uses eggshell features as an all-or-nothing cue," the researchers write. Contrary to a common scientific assumption, the mockingbirds' decisions weren't purely based on the degree of difference between their eggs and foreign eggs. "Instead, this host rejected brown eggs but accepted equally dissimilar blue-green eggs," they write. These patterns suggest important and unexplored aspects of co-evolutionary dynamics," both in the cowbird-mockingbird relationship "and host-parasite dynamics more generally."

More research is needed, Hanley and his colleagues add, to reveal how these birds influence each other's evolution. In the meantime, countless cowbirds and other brood parasites will continue to be raised by unwitting foster parents, while countless hosts will continue pushing their brains to spot the intruders before it's too late. As Stoddard recently told Science magazine, "What's going on in the brains of [birds] is even more complex and interesting than we imagined."

These eggs shed light on a battle of wits between cowbirds and mockingbirds
A new study looks at the co-evolutionary 'arms race' between a brood parasite and its hosts.