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Earth Notes: Robins Know Their Eggs

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For many people, a nest full of blue robin eggs is the perfect symbol of spring. But, do robins recognize their own eggs? That might sound like a silly question, but it’s an important one.

Parasitic cowbirds lay their eggs in robin nests in hopes of scoring free childcare. They rely on others to raise their young. Robins - including the American robin of the Colorado Plateau - stay alert to these uninvited guests. They constantly clean out broken shells and debris left behind by the lazy cowbirds. But what makes robins able to recognize objects that aren’t theirs?

Biologists at the University of Illinois decided to find out. They 3-D printed bright blue plastic eggs and tucked them into robins’ nests. They all matched the color of a real robin’s egg, but they varied in shape from oval to diamond and everything in between. The scientists came back later to check on the nests, and see if the robins had tossed them out, like a strange Easter egg hunt in reverse.

Some of the results were surprising. All of the robins continued to care for their real eggs, but they didn’t always spot the fake ones. The skinnier the object, the more likely they were to reject it – even though real eggs can be skinny, too. The robins were less likely to throw out eggs with sharp edges.

Researchers speculate the robins kept the sharp-edged 3-D eggs because they hadn’t evolved to deal with an egg shape not found in nature. However, they were quick to spot difference that DO occur in the real world.

Melissa joined KNAU's team in 2015 to report on science, health, and the environment. Her work has appeared nationally on NPR and been featured on Science Friday. She grew up in Tucson, Arizona, where she fell in love with the ecology and geology of the Sonoran desert.