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Life can be tough for some parent birds. Imagine the horror of waking up one day to find that the chick in your nest wasn’t yours, but plonked there when it was still an egg by another profiteering bird species too lazy to build its own nest.
For decades, researchers have been trying to find out how birds avoid scenarios like this, by identifying then ejecting rogue imposter eggs from their nests. But there’s a problem: using real eggs raises ethical issues, and it can be difficult to make imposter eggs from plastic, wood or plaster-of-paris without imperfections.
In a new study published in the journal PeerJ, researchers turned to 3D printing eggs to investigate why birds reject some and not others. In the video accompanying the study, researchers use a MakerBot 3D printer to create the egg layer by layer. The surfaces were then sanded down and placed in acetone (nail polish remover) to ensure an extra smooth surface before painting.
“Brood parasites” are birds that lay eggs, then casually deposit them into other birds’ nests, leaving them to do all the hardcore parenting work. Sometimes unsuspecting foster parent birds can raise these invader eggs at the cost of their own chicks’ lives. This has incited a kind of evolutionary arms race among some bird species, who have become pros at using markers such as egg size, colour, shape, and patterns to identify and purge rogue eggs from their nests.
“Hosts of brood parasites vary widely in how they respond to parasitic eggs, and this raises lots of cool questions about egg mimicry, the visual system of birds, the ability to count, cognitive rules about similarity, and the biomechanics of picking things up,” said brood parasitism expert and chair of the Biology Department of Bates College Don Dearbon in a press statement.
The researchers used 3D printing to examine how American robins, Turdus migratorius, rejected the eggs of a brood parasite known as a brown-headed cowbird. While robins lay blue-green eggs, cowbirds lay smaller, beige eggs with spots.
“This study uses 3D printing for a more nuanced and repeatable egg-making process, which in turn will allow more refined experiments on host-parasite coevolution,” said Dearbon.
The researchers created digital egg models of the cowbird, and printed them in ABS plastic. According to the researchers, 3D printing not only gets the shape and form of the eggs down to a tee, it also allows them to fill the egg replicas with gel and water, meaning that they capture the thermodynamic properties of real eggs too.
The 3D-printed eggs were either painted blue-green to match the eggs of their American robin hosts, or beige to match the cowbird eggs. They were placed in the American robin’s nest for six days and monitored to see how the birds would react. The study found that robins accepted 100 percent of their own egg replicas, but ejected 79 percent of the imposter eggs.
According to the researchers, these results are actually pretty similar to past conclusions drawn from plaster-of-paris eggs; however, they noted that 3D-printed eggs have an added advantage of being less variable, and easier to create in any shape, size or form. Digital models can also be shared more widely among researchers determined to get to the bottom of why host birds purge some eggs and not others.