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From baby blue to black-speckled beige, bird eggs are as varied as the species that lay them. Why bird eggs come in so many colors is still something of a mystery, but new research identifies a reason for a common trend among birds, especially those up north: their dark-colored eggs.

Previous research on tropical birds has shown that darker hues help guard against predation — they are harder for hungry neighbors to see. Deep pigments in eggshells also contai