It seems birds know a thing or two about sun screen, using just the right SPF number on their egg shells to protect their embryos.

A new University of London study published in the journal Functional Ecology suggests that birds use adaptations in both color and shell thickness to let the perfect amount of sun light reach their youngins' inside the egg.

The research team used the museum eggs of 75 bird species to measure how much light could penetrate the egg for a given species.

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They found that egg shells got the right amount light they "needed," based upon their exposure to ultraviolet rays. For example, the eggs from birds that nest in holes or deep cavities had very white shells that let in as much light as possible. By contrast, they found, birds that nest out in open areas with more direct sun exposure use extra pigment so the sun light can be dialed back.

Duration of exposure mattered too, the team learned. Birds with long incubation periods need thicker shells and more pigment for protection, so that's what they have adapted for themselves.

"Embryos do need UV exposure to develop. Too little and they don't develop enough ... too much and it causes damage," team member Dr. Steven Portugal told BBC Nature. Luckily for the birds, they're well aware of the danger and seem to have tailored their eggs to moderate it.