The trouble with eggs

THE bird maternity season is underway and, like expectant fathers, people worry about the nests in their gardens. A neighbour, mowing her lawn, frightened a blackbird off its eggs.

The bird did not return for 20 minutes. Could the eggs have chilled, she asks? Might her gardening activities cause the bird to desert the nest? These are interesting questions and difficult to answer.

Birds are not rational creatures which make conscious decisions. Their behaviour is automatic, refined by natural selection over countless generations. Some individuals are more temperamental than others, but the responses of most birds to danger are fairly consistent. Desertion, for example, is most likely to occur during the early stages of nesting. Excessive disturbance, or harassment by predators, can cause a bird to abandon a nest and it makes sound ecological sense for it to do so.

A nest in a high risk location is unlikely to be successful and a bird will do better elsewhere. As more eggs are laid and incubation gets under way, however, desertion becomes less likely and that’s hardly surprising when you consider the huge investment which the bird will have made.

The avian egg is a remarkable invention. Flying animals must be as light as possible; by laying eggs, a bird won’t have to carry embryos around with it. But there’s a price to be paid; an egg must be self-sufficient when it’s laid, with enough nourishment within it to sustain the growing chick until hatching. Gathering the extra food needed to form eggs is demanding. The weight of a blue tit’s egg is almost one-tenth that of its mother. No human expectant mother could put on a stone of body weight in 24 hours, but a blue tit does the equivalent every day for a fortnight. The completed clutch weighs more than the mother! Having done all that work to form eggs, a bird is unlikely to desert them. Once there are young in a nest, desertion is even less of an option.

The second major problem with eggs is that, once incubation starts, they must be kept warm. If they become chilled, the chicks within them will die. Small objects lose heat much more quickly than large ones, so tiny birds make snug, well insulated, nests. The most elaborate one, in our part of the world, is that of the long-tailed tit. It’s a domed structure of moss cobwebs and hair, covered on the outside with camouflaging lichens. The chamber is lined with feathers; nests examined in Britain had between 1,500 and 2,100 individual feathers. It takes the adults between two and a half and five weeks to complete the work. Thanks to the strands of spider’s web incorporated in it, the nest is elastic and expands as the baby birds grow inside it. There can be up to 12 youngsters and, with their long tails, they will need more space as they grow.

The wren, slightly bigger than the long-tailed tit, makes a thick-walled ball of moss, with an entrance at the side and well insulated to save on heating bills. Though not as luxurious as the long-tailed tit’s, it’s still an extraordinary creation. The wren’s home may not rate a five star classification, but it certainly deserves four.

A blackbird is about 10 times heavier than a wren. Its eggs, being bigger, are not as vulnerable to chilling. The bird can get by with a modest structure in the three star league. The open cup affords less protection from the elements and, when it rains, the roof will be the mother bird herself.

Your average wood-pigeon, four to five times heavier than a blackbird, can afford to nest at the budget end of the market. The modest platform of loose sticks just about deserves a one star rating. But there are birds which don’t build a nest at all. The oystercatcher, for example, which weighs about the same as a wood-pigeon, lays its eggs in a depression on the ground. A few pebbles may be added to help with camouflage, but no nest is built. The oystercatcher is a large wader, but even small waders don’t go in much for architecture. The ringed plover, for example, lays its four eggs on a shingle beach. The chilling problem is less acute, because waders lay larger eggs, in proportion to their body weight, than do songbirds. The ringed plover is about twice as heavy as a sparrow, but its eggs are five times heavier. Shingle, during daylight hours when the sitting bird is most likely to be disturbed, is quite a warm habitat, as the sun tends to roast the stones.

Before incubation starts, eggs are indifferent to cold. However, once the adult sits and the embryo begins to develop, there is no going back. Body temperature must be maintained until the baby hatches. Eggs gradually lose moisture during incubation. Fluids are a good heat retainer and an egg, which has lost some of its moisture, will cool more quickly, but there are compensating factors. As a chick grows it becomes hardier and the babies of most ground nesters grow a downy coat while in the egg.

Eggs, though fragile, are remarkably resilient, so there is no need to panic if you disturb an incubating bird. It is important, however, not to interfere with a nest, disturb the surrounding vegetation or leave tell-tale signs for predators. It is best to leave the vicinity as soon as possible.