New York: People have always wondered why many birds lay bright blue eggs. Researchers have now shown that the colour could protect the embryo from harmful sunlight, including dangerous ultraviolet (UV) radiation. David Lahti of the City University of New York and Dan Ardia of Franklin & Marshall College tested the hypothesis that pigmentation might help an egg strike a balance between two opposing and potentially damaging effects of the sun - light transmission into light-coloured eggs, and heating up of dark-coloured eggs. As predicted, more intensely blue eggshells shielded the interior from light, including dangerous UV radiation, but more intense colour also caused eggs to absorb more light and heat up, which can be even more dangerous in brighter environments. These two patterns - termed by the authors "pigment as parasol" and the "dark car effect" --combined with a knowledge of the nesting behaviour and habitats of birds, can lead to predictions as to why the eggs of some birds vary across species from blue to white. Darker eggs are predicted in moderate light to shield the embryo, but in brighter nests the dangers of egg heating predict lighter coloured eggs. Whereas camouflage from predators is still probably the single most important factor governing the evolution of dull and mottled egg colours, for the brighter colours the biophysical evidence points to the sun, the study said. The findings appeared in the journal The American Naturalist.