''The limits of egg recognition: testing acceptance thresholds of American robins in response to decreasingly egg-shaped objects in the nest

A study by Mark Hauber, a professor of evolution, ecology, and behavior at the U. of I., and colleagues reveals what robins thought of various dimensions of fake plastic eggs inserted into their nests. The researchers say 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 pointy-sided eggs that were closer in width to their own eggs.

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https://neuroscience.illinois.edu/news/2021-01-29t191557/how-eight-sided-egg-ended-robins-nest