Published: 27 June, 2002

The great blue mystery of robins' eggs

Why are robin's eggs blue?

More than 30 years ago, famous bird biologist David Lack wondered the same thing. He noticed that birds nesting on the ground often lay brown or gray eggs that blend in with the surroundings. And birds that lay highly visible white eggs often conceal their nests in a hole. He suggested that the blue eggs of many songbirds might blend in at the bottom of a dark nest that is screened by leaves from above. This could help hide the eggs from predators such as squirrels, ravens, and jays. But, he admitted, blue eggs were the "biggest puzzle."

Franz Götmark tried to solve the puzzle by painting quail eggs white, blue, or spotted, then putting them in artificial nests in trees. Predators took all three types equally. So although, in theory, blue eggs in nests might be hard to find, scientists haven't yet come up with the evidence.

And that's just one of the mysteries when it comes to the color of eggs. Why are an emu's eggs dark green like an avocado? Why are a Chilean tinamou's eggs purplish? I don't know.

Most birds have pigment glands that deposit colors on the egg as it passes through the oviduct. Usually each bird species has a characteristic egg color.

But in some species, such as the common murre, different females lay eggs with different colors—for example, bright pinkish, creamy or pale blue. Additionally, the eggs can have black, red, or yellowish-brown blotches or squiggles. A murre lays her eggs on a ledge near the eggs of other females. Distinctive eggs may help her recognize which ones are hers. Experiments show that a murre will only accept a new egg if it resembles her own.

OK, but why do the eggs laid by the same American crow look so different from one another—some heavily spotted, others nearly plain? Maybe some day you can help us find out.