By Charlotte Owen
"A box without hinges, key or lid yet golden treasure inside is hid.” This was one of the ‘tricksy’ riddles that appeared in Tolkien’s The Hobbit and the answer is, of course: an egg.
Bilbo was thinking specifically of a bird’s egg when he set this riddle, referring to the hard outer shell, but birds owe their success to their reptilian ancestors. The origin of the eggshell dates back to the dinosaurs and beyond, when the ground-breaking evolution of a leathery protective coating freed the first reptiles from a reliance on wet and watery breeding grounds. With their eggs now safe from the risk of drying out, they waved goodbye to their amphibian cousins and went on to conquer the land and sky.
Tufted duck eggs
Their avian descendants have since perfected the egg and it would be difficult to design a better form of natural packaging for a developing embryo. The shell is tough enough to provide protection yet permeable so that the chick inside can breathe, and it also provides a vital supply of calcium for growing bones. The “golden treasure inside” is the egg yolk, a high-calorie food source full of vitamins and minerals, which is cushioned by the egg white – or albumen – which protects the yolk and provides the chick with essential protein. All that remains for the parents to provide is warmth.
Many birds will now be sitting on eggs and these natural treasures come in a huge variety of colours and patterns. Hole-nesting birds like the Blue Tit lay pale, white-ish eggs that stand out in the gloom of the nest cavity, helping the parents to avoid accidentally crushing them. Ground-nesting birds like the Ringed Plover rely on excellent camouflage and generally lay brown, speckled eggs that blend in with their surroundings.
Ringed Plover eggs
But the well-hidden eggs of the Blackbird and other species that nest in dense vegetation can afford to be more colourful. They are usually bright blue and this aids temperature regulation, with more intense pigmentation allowing eggs in the shade to absorb and retain more heat - a useful ability when your nest is tucked in the heart of a hedgerow.