HOW EIGHT-SIDED “EGG” ENDED UP IN A ROBIN’S NEST
In the 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 3D-printed by the lab of Mark Hauber, a professor of animal behavior at the University of Illinois and a fellow at Hanse-Wissenschaftskolleg in Delmenhorst, Germany.
He and his colleagues reported that the narrower the fake eggs got, the more likely the birds were to remove them.
But 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 not.
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. 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, 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.