The Wonder of Wildlife - The mystery of bird eggs

I was on my way to the hen house the other night, when I spotted half an empty egg shell on the path. No, it wasn't one of Little Brown Hen's. This was a small egg, pale and finely freckled with reddish spots.

It lay on the ground, just in front of the potting shed - which gave me a clue as to who had laid it. Or indeed, who had just hatched from it.

A robin has made his nest in the ivy which climbs thickly between the shed and the wall. He watches me daily, whatever I am doing in the garden. His greatest hope is that I will raid the compost heap - exposing any number of worms and grubs for him to feed his family. And he's surprisingly tame. Hopping about around my feet, or perching on the bean poles, head on one side.

Then I remembered I had two books on bird eggs. One, which belonged to my father, dates from 1924. Its pages are thick and slightly yellowed, with one blurry colour illustration. Even though it is nearly 100 years old, it’s a brilliant reference book for garden bird lovers. My only sadness is that many of the birds are described as 'commonplace': corncrake, nightingale and turtle dove are rarely seen or heard now.

My robin, I learned, builds his nest from late March onwards, raising 3 broods a year - sometimes into winter. His nest is "rather large for his size, made of leaves and moss, lined with hair." Instead of hair, I've seen my little chap plucking strands from an old, loose, wool rug I made from one of my sheeps' fleece, long since discarded because it bought moth into the house, and now hangs over the gate into the paddock.

The book also describes the egg colour - "White, finely freckled or spotted with pale red. In some cases this freckling is so fine and uniformly distributed as to give a pale reddish tint to the whole egg." Bang on.

This led me to my second book - The Most Perfect Thing: Inside (and Outside) a Bird's Egg, by Prof. Tim Birkhead. Page after page brings fascinating facts. If only school biology classes were so gripping.

I’ve learnt how egg shells are constructed to be waterproof, but at the same time allow the developing embryo to breathe. A kind of biological gore-tex. That the tiny goldcrest (who weighs less than a teaspoon of sugar) keeps her clutch of eggs warm by increasing the blood flow into her spindly, thin legs so they act like little immersion heaters inside the mossy insulated nest.

Analysis of the egg white and yolk each get a chapter (you’ll never eat a boiled egg again with indifference), and we follow the whole complicated process of egg formation within the bird. An oological miracle. Sadly, the learned Prof doesn't answer which came first, the chicken or the egg, but he does tell us which end of the egg comes out first, the blunt or the pointy end. (I’ll let you buy the book to find out…..)

Why, and how, are birds' eggs are so varied in their colours and marking? From the soft freckled blue of the blackbird to the almost pure white of the wren, and the Jackson Pollock scribbles of the yellow hammer - scientists have hypothesised through the centuries as to why they are so varied.

Is it camouflage? Ground nesting birds like the lark or the tern lay heavily patterned, earth-coloured eggs to fool predators. But this doesn't explain the shocking blue of the song thrush egg. Nor why the eggs laid later in a clutch often have paler shells than their siblings'. Prof Birkhead not only explores each theory (the paler eggshell allows more light through to the embryo, and therefore helps the eggs which are laid later to hatch sooner) but even sets up his own experiment to see if he can understand how the markings got onto the shell. This involved felt tips, a hen's egg, clamps and a lot of rotating. It wasn't very successful.

I urge you to buy Birkhead's book. You may not find a robin's egg at your feet, but it will make you think before you crack open your breakfast treat….

More about Sarah...
Sarah has been growing organically for over 30 years. Gardening has always run alongside her career in museums and climate change studies, whether it was growing salad leaves in pots on a London windowsill, tomatoes in a tiny flat in Hong Kong, or her present plot in rural Oxfordshire. She shares this space with hens, two (very fat) sheep, two cats, a husband and any wildlife which wanders in. Foxes, badgers, kites, owls, moles and buzzards are regular visitors – as are bats, butterflies and bees. She readily admits that being outside, helping things to grow, keeps her sane and completely content.