Eggshells act like 'sunblock', study suggests

Song thrush eggs Blue, white or spotted: Eggshell pigmentation may act as 'sunblock' controlling the light reaching embryos

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The eggshells of wild birds may act like "sunblock", scientists have said.

A range of UK birds' eggs showed adaptations in pigment concentration and thickness to allow the right amount of sun to reach the embryos inside.

Researchers examined 75 species' eggs kept in a museum collection.

"Embryos do need UV exposure to develop - too little and they don't develop enough... too much and it causes damage," said team member Dr Steven Portugal from the University of London.

"Birds whose nests are exposed to the sun and birds which have long incubation periods too, have more pigment and allow less light to go through the shell to avoid UV damage to embryos," he explained.

The study, published online in the journal Functional Ecology, suggests thickness and pigment in eggshells change depending on the nest environment.

Wild birds' eggshell colours can be white, blue and spotted. The blue colour found in many eggs is caused by a pigment called biliverdin, while dark spots are produced by a darker pigment, protoporphyrin.

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The findings may shed new light on the colour variations found in wild birds' eggs.

"Within the UK you can have species like stone curlews, oystercatchers, Arctic skuas and nightjars that nest out in the open on the ground, essentially exposed to the elements, including the sun and damaging UV," Dr Portugal told BBC Nature.

These species' eggs contain extra pigment which, according to the new research, helps control the amount of light entering through the shell and reaching the embryo.

Other eggs belonged to species that nest in holes, burrows and cavities, such as hoopoes, little owls and green woodpeckers.

The "immaculate" white shells prevalent in these nests allow "greater light transmission through the shell to assist embryonic development under low-light exposure," said Dr Portugal.

The study also showed that birds with longer incubation periods had thicker shells with more pigment, to protect against harmful UV rays.

To examine how different eggshells control the amount of light allowed to shine through, Dr Portugal and an international team of researchers looked at eggs kept in the Natural History Museum in Tring, Hertfordshire.

Using a spectrophotometer, they shone a light through eggshells that had been cut in half and measured their "resistance".

"One hundred percent light transmission means nothing in the way at all," explained Dr Portugal. "Fifty percent would mean half the light is blocked by the shell."

The team also extracted and examined the pigment present in each shell and measured their thickness.

Previous studies have shown an array of adaptations among birds' eggs. These include many eggs displaying dark spots (or maculation) for camouflage, and pigment which can fight infection to protect embryos.

Dr Portugal added: "Eggshells are complex structures and far more sophisticated than many people would realise or appreciate."

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