The Wonder World of Nature [antikvár]

The swallow is possibly the best known of our migrants, being found nesting at farms throughout the country and in the remotest places. The bird winters in South Africa, arriving in Britain in March and April. Lt leaves again from September onwards.BÍRD MIGRATIONTHE ONLY certain thing known about migration is that birds migrate; as to why birds migrate, or how they find their way, we are nőt much better informed than the one-time Bishop Godwin of Hereford who thought they migrated to the moon.Gilbert White, the English country parson who has been rightly styled the father of modern ornithology, was aware that birds migrated, but he thought aiso that somé of them hiber-nated. He was convinced this was so in the case of swallows. Every modern schoolboy knows diflerently; yet in the past few years birds of the swallow family have been found in a torpid state in America. The extent of this habit is nőt known.Many theories have been put forward to explain the migratory ürge in birds. One such theory, for long a favourite, was called the glacial theory. This held that birds were driven south by the creeping ice cap, then returnednorth as it retreated. For a number of reasons, this theory has been rejected.Food supply available in winter may be an important factor, but it cannot explain the whole mystery. It seems reasonable to argue that birds whose food supply is cut off should move south to where it becomes available again. Yet the fact remains that individuals of certain species migrate, while others of the same species stay where they were born all the year round. Even individual swallows have been known to stay in the south of England through the winter.TRUE MIGRATIONMigration in the strict sense must not be confused with " dispersal " or " irruption." Birds which merely disperse over a wide area of the country after the breeding season are not true migrants. Similarly when bands of birds like crossbills appear suddenly in thisWWN9a2