Day and night
The sun is probably the most important signpost for migrating birds. But birds have to adjust their direction as the sun crosses the sky from east to west. A bird flying north must keep the sun behind it. But to stay on course, it must also make sure that the sun is over its right shoulder in the morning and over its left shoulder in the afternoon. Birds also know that the height of the midday sun gets lower the further north they travel. This helps them to recognise when they have reached their destination. Birds use an internal ‘clock’ to make all these calculations. Their brains can calculate exactly how long the sun takes to pass across the sky at different latitudes and at different times of year. During clouds and bad weather, birds can still find the sun’s position by detecting polarised light. This consists of bright ultra-violet rays that humans cannot see. The rays reveal where the sun is, even if it is hidden by heavy cloud. But birds are not perfect, and many more migrants get lost during cloudy weather. Of course the sun is no help at night. Night-flying migrants usually set off at sunset, so they can get their first directions from the setting sun. They then follow the positions of the moon and stars. In one experiment, scientists allowed birds to fly round inside a planetarium. As the artificial stars moved around the artificial sky, the birds adjusted their positions to keep on course – just as though they were on a real migration. |
