ATTENBOROUGH'S WONDER OF EGGS

For BBC Natural World

David Attenborough reveals what lies behind nature’s most perfect thing: the bird’s egg. The chicken egg is what many of us see each morning at breakfast. But our familiarity with this common kitchen article blinds us to the wonder of the eggs that are produced by the ten thousand different species of bird that are alive in the world today. Each one serves a single purpose: to nurture new life. But there is an astonishing diversity of size, shape, colour and pattern.

In the woods of Oxfordshire, great tits and blue tits gather calcium from fragments of snail shells in order to build their eggshells. But forming the shell isn’t the first stage of an egg’s existence. Fertilization in birds is different from human conception: it doesn’t occur within hours of insemination. Instead, female birds store sperm, sometimes for weeks, during which time they build up the yolk upon which a tiny disc, containing all the female genetic material for the embryo, sits. The yolk provides all the nutrients the developing embryo will need. In the final, miraculous stages of egg formation, albumen is added to provide the embryo with water and then calcium is pumped around it to form the hard shell. But which end will emerge first from the bird? A study conducted in the 1800s, involving a chicken and a pencil, reveals that the chicken lays its egg blunt egg first.

The egg is adapted to almost every environment, from the poles to the tropics. But one element crucial to every egg is heat. In a Welsh churchyard, a goldcrest, reveals a surprising incubation strategy: she warms her egg with her legs. The great tits of Oxfordshire have a different strategy: they adjust the amount of time they incubate their eggs in order to coincide their chicks’ hatch date with a glut of food.

Inside the egg, the embryo has started to breathe. But the pores that allow the oxygen in also leave the egg vulnerable to invasion by microbes. On the edges of a tidal lagoon in Dorset, the eggs of mute swans risk attack from bacteria in floodwater. But a microscopic protective outer layer on the shell, combined with the anti-microbial albumen within, provide a strong defence.

The outside of the egg, too, is an important part of its design. In the Fens of East Anglia, Professor Nick Davies reveals how the colour of a cuckoo’s egg plays a crucial part in its trickery. By mimicking the colour of another bird’s egg, the cuckoo deceives others into raising its chick. New research conducted at Princeton University, America by Dr Mary Caswell Stoddard reveals how the evolutionary arms race between cuckoo and victim has escalated to a more sophisticated level than previously thought.

On the Welsh island of Skomer, Professor Tim Birkhead unravels one of the avian world’s greatest mysteries: the extreme egg shape of the guillemot. Many theories have tried to explain why it is so curiously conical. One of the most seductively simple is that its shape stops it from rolling off the cliffs where guillemots breed. But Tim’s research has shown the real reason might be muck: a guillemot colony is a dirty place and the shape may have evolved to keep the blunt end of the egg, where the chick’s head lies, out of the mess. At Princeton University, Dr Mary Caswell Stoddard’s research shows a link between egg shape and flight. But the puzzle of egg shape is far from complete: future research will probably show that it’s incubation that determines egg shape.

By creating an egg, natural selection devised the perfect life-support system. But its success will depend on its destruction. The shell, strong enough to protect the chick, must also be weak enough for it to break free: the chick has absorbed calcium from the shell into its skeleton to make it stronger and the shell weaker. Each successful hatching is confirmation that eggs are miracles of evolutionary design.

Back to top