WHY do birds’ eggs have patterns on them? In some cases the answer seems pretty obvious. Among the waders, such as curlews and snipe, most of which nest on the ground, the pigmentation of the eggs provides protective colouring, and the eggs blend into their surroundings so that crows will not see them and steal them.

But the zoologist Andrew Gosler was puzzled by the number of eggs of different species with colouring that seems to have no protective purpose, and he has now come up with a completely different answer in the case of those birds. He has found that the pigment strengthens the eggshell.

He was studying great tits in that famous wood near Oxford, Wytham Wood, where several hundred pairs of great tits nest in boxes put up by the Oxford ornithologists. Great tits have eggs with pink spots, but Dr Gosler found that they vary greatly in the number and colour-intensity of the spots. He also knew something that might at first have seemed a wholly unconnected fact — that female great tits eat small snails and their shells in order to get calcium for their own eggshells.

His remarkable discovery was that great tits nesting around a limestone hill at one end of the wood, where there were more and larger snails, not only produced thicker eggshells, but also were the birds that had eggs with the least pigmentation.

Great tits nesting on the clay soils containing less calcium had distinctly thinner egg-shells — and the most numerous and most colourful spots. From there he went on to demonstrate that the spots occurred precisely where the shell was at its thinnest, strengthening it at its weakest points.

So the pink pattern on the great tit eggs is not a signal of any kind. It is as incidental to any practical purpose as the pattern of the mortar in a brick wall. The pigments, as Dr Gosler puts it, are “a very sophisticated mortar holding together the calcite ‘bricks’ of the eggshell wall. Any pattern they produce is secondary to that.” This is a discovery that, he thinks, will prove to apply to the eggs of many small birds, such as the eggs of wrens and swallows.

Dr Gosler’s arguments can be found in the July number of British Birds magazine (www.britishbirds.co.uk). This is a remarkable piece of work, turning established ideas about birds’ eggs completely upside down.

WHAT TO LOOK OUT FOR

Birders Look out for hawking hobbies.

Twitchers semi-palmated sandpiper at Back Saltholme Pools, Cleveland; male pine grosbeak near Ongar, Essex; great white egret at Mockbegger, Hampshire.

Details from Birdline, 0906 8700222 (60p a min) www.birdingworld.co.uk; www.rspb.org.uk

derwent.may@thetimes.co.uk