Driven to Discover

Why are robins’ eggs blue?

Blue eggs may be adaptive.

Robert Zink, Breckenridge Chair in Ornithology

It is tempting to assume that robins’ eggs are blue for some reason related to their current environment. However, closely related species also have blue eggs, so the question actually becomes one for evolutionary analysis, and what needs to be answered is this: Why did ancestors of robins evolve blue eggs in the first place, and why are they still blue? That is difficult to answer because robin ancestors likely lived several million years ago. However, the fact that blue egg color has been retained over evolutionary time, and it is shared among many species, suggests that there is an adaptive reason for its continued existence. Some recent studies suggest that blue eggs in a nest may be cryptic and provide camouflage by imitating spots of light on green leaves. But it would be difficult if not impossible to know whether this was the original reason blue eggs evolved. Also, a recent experiment found that predators find nests irrespective of egg color. So, it is also possible that blue eggs are an evolutionary hand-me-down’ and that nothing in the current environment either favors blue or works against it. Still, there are no definitive studies that have provided an answer.

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