Last spring, robins living on an Illinois tree farm sat on some unusual eggs.
Alongside the customary brilliant blue ovoids they had laid were some unusually shaped objects. Although they had the same color, some were long and thin, stretched into pills. Others were decidedly pointy — so angular, in fact, that they bore little resemblance to eggs at all. If robins played Dungeons and Dragons, they might have thought, “Why do I have an eight-sided die in my nest?”
The answer: Evolutionary biologists were gauging how birds decide what belongs in their nests, and what is an invasive piece of detritus that they need to throw out.
Thanks to the results of this study, published Wednesday in Royal Society Open Science, we now know what the robins thought of the eggs, which were made of plastic and had been 3-D printed by the lab of Mark Hauber, a professor of animal behavior at the University of Illinois, Urbana-Champaign and a fellow at Hanse-Wissenschaftskolleg in Delmenhorst, Germany.
He and his colleagues reported that the thinner the fake eggs got, the more likely the birds were to remove them from the nest. But curiously, the robins were more cautious about throwing out the pointy objects like that eight-sided die, which were closer in width to their own eggs. Birds, the results suggest, are using rules of thumb that are not intuitive to humans when they decide what is detritus and what is precious cargo.
It’s not as uncommon as you’d think for robins to find foreign objects in their nests. They play host to cowbirds, a parasitic species that lays eggs in other birds’ nests, where they hatch and compete with the robins’ own offspring for nourishment. Confronted with a cowbird egg, which is beige and larger than its blue ovals, parent robins will often push the parasite’s eggs out. That makes the species a good candidate for testing exactly what matters when it comes to telling their own eggs apart from other objects, Dr. Hauber said.
The researchers 3-D printed two sets of decoy eggs. One group got progressively thinner, and the other got more and more angular. They carefully painted them robin’s egg blue, so birds could rely only on shape to tell the difference. Then they deployed these fake eggs in nests scattered around tree farms. As they revisited the nests, they kept track of which shapes had been removed by the nest’s owner.
The robins had a good eye for eggs that were too thin. Eggs that were about 75 percent the usual width were accepted more often than they were rejected. Eggs that were less than 50 percent the usual width were almost always kicked out.
But the pointed objects were not rejected nearly as often. In fact, only the very pointiest decoy, the eight-sided die shape, got pushed out almost every time.
“It was very surprising for us,” Dr. Hauber said.
The birds seem to be responding to variables that matter in nature. Cowbird eggs, for instance, are noticeably wider than their own, so robins may have evolved a canny sense of when width is off.
“They seem to be quite hesitant about rejecting eggs when the variable that we changed was not natural,” Dr. Hauber said, referring to the angular, pointed eggs. “Robins don’t know what to do with it, because they’ve never evolved to respond to it.”
And rather than toss out one of their own eggs by mistake, they let it lie.
Identifying what screams “egg” to a bird is an intriguing way into what the world looks like to other animals, but it may also be useful in future studies of avian brains.
“Like humans have a face recognition center in the brain, birds have an egg recognition center in the brain,” Dr. Hauber said. He hopes that M.R.I. studies can help reveal what goes on neurologically when birds see such an important object. These peculiar plastic eggs, progressively more different from the robin’s own, may be valuable tools in lab experiments probing what birds are doing when they look at their nests.