3-D printed cowbird eggs were used to trick robins. (Mark Hauber. CC BY 4.0)

Not all birds care for their own young. Some of them are brood parasites, dumping their eggs into the nests of other birds in the hope that some other avian will do the rearing. To study how different bird species respond to the presence of alien eggs, researchers have to make replicas of the eggs that might get dropped into their nests naturally. According to a new paper, 3-D printing may be the ersatz egg technique that scientists have been waiting for.

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In a study published Tuesday in PeerJ, scientists studied American robins and the species' brood parasites, brown-headed cowbirds. The two species have very different eggs -- the robins' are blue and the cowbirds' are beige -- but cowbirds fool robins enough for their parasitism to be an evolutionary win. The researchers wanted to see how important the shape and color of an individual impostor egg was to its survival in the robin nest.

As with all egg-rejection studies, they needed to create fake eggs for the robins to accept or snub. But instead of using the usual mediums of wood or plaster, the group decided to 3-D print them from plastic. This way they could create uniform eggs with less room for human error. They could also leave them hollow and fill them with gel for a more realistic feel.

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The new study didn't find any significant changes in egg rejection rates when they subtly tweaked the appearance of the fake cowbird eggs. But they did find that the overall rejection and acceptance rate of the fake cowbird and robin eggs was in line with more traditionally produced fake eggs: The robins accepted 100 percent of the eggs made to look like their own brood, but accepted only 21 percent of the fake cowbird eggs.

In the future, researchers will be able to use the tech to create uniform differences from one set of eggs to the next -- which is important if they want to study how minute changes in a parasite's egg can affect its success.

3-D printed eggs could also hypothetically be made to have pierceable shells, which is actually quite important: In the study, the authors write that many birds reject foreign eggs by piercing the shells to kill the embryos within. If a bird can't pierce an egg (because it's made of wood or plaster) it might skew results by allowing it to live despite intentions otherwise. Or a frustrated bird might pack up and abandon the entire nest, killing their own babies in the process. They haven't created it yet, but the authors imagine an egg design that uses a shell-like outer layer to protect a gooey center, creating the illusion of a real egg if pierced.

"3-D printing technology is not just in our future -- it has already revolutionized medical and basic sciences," senior study author Mark Hauber, an animal behaviorist at Hunter College of the City University of New York, said in a statement. "Now it steps out into the world of wild birds, allowing standardized egg rejection experiments to be conducted throughout the world."

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