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.