Bird migration is the seasonal movement of birds from one region or climate to another and back again.   Chicago is an important stopover on the Mississippi Flyway. Birds use its lakefront, ponds, and parks as resting and feeding stops during migration. Some birds even make their homes here. During migration, birds have to pass between Lake Michigan on the east and open farmland on the west and to the south. Chicago habitats provide much needed trees, shrubs, and flowers, which are not available in open farmlands.   Birds are the most mobile creatures on earth and approximately 7 million, encompassing over 300 species, pass through Chicago each year.		During long flights, birds may be overcome with exhaustion, so Chicago is an important resting place. Other dangers include storms and aerial obstructions such as tall buildings and radio and TV antennas.