Migration: A semi-annual event
Most species used established routes for migration and travel more or less on schedule, arriving and leaving regularly. Migration, breeding, and moult are phases in the annual cycle of birds that are regulated by the endocrine system. Migration is a semiannual event, dependent especially on the pituitary gland in the brain. Usually prior to migration fat reserves, not present at other times, are accumulated rapidly for extra fuel during the long flights. Also, many strictly diurnal birds become nocturnal during migration. Seasonal differences (photoperiodism) influence migratory behaviour of some northern species. Generally birds migrate close to the earth's surface, although some birdspecies may migrate at more than 1 km altitude. Most birds migrate at between 50 and 80 km/h and stop and feed as they proceed along the migration front. Although some birds use obvious landmarks such as coasts, rivers and mountain ranges other birds will migrate without the aid of directional features. Evidence suggest that migration in daytime is guided by the position of the sun and at night by the patterns of stars. This would necessitate that migrations need to be done on clear nights. On cloudy nights birds tend to get lost and if they are released in a planetarium where the constellations have been rotated so that they no longer match the position of the stars in the heavens, the birds will orientate with the visible, artificial constellations. Still other birdspecies appear to be able to use the earth's magnetic field as a guide.