Bird Migration
Bird migration is the regular seasonal journey undertaken by many species of birds. Bird movements include those made in response to changes in food availability, habitat or weather. Sometimes, journeys are not termed "true migration" because they are irregular or in only one direction (movement of young away from natal area). Migration is normally anually. In contrast, birds that are non-migratory are said to be resident or sedentary. Approximately 1800 of world's 10,000 bird species are long-distance migrants. Birds may often fly in large packs when migration to avoid predators, have a sense of unity etc.
Some migration routes are shown below:
Other pictures:
