Although no one is certain why migration occurs, there are several theories. One theory claims that prehistoric birds of the Northern Hemisphere were forced south during the Ice Age. As the glaciers melted, the birds came back to their homelands, spent the summer, and then went south again as the ice advanced in winter. In time, the migration became a habit in spite of the disappearance of glaciers. Another theory proposes that the ancestral home of all modern birds was the tropics. When the region became overpopulated, many species were crowded north. During the summer, there was plenty of food, but during the winter, scarcity forced them to return to the tropics. A more recent theory suggests a relationship between increasing daylight and the stimulation of certain glands(腺) in the birds' bodies that may prepare them for migration. One scientist has been able to cause midwinter migrations by exposing birds to artificial periods of daylight. He has concluded that changes occur in the bodies of birds due to seasonal changes in the length of daylight. According to one theory, when the glaciers disappeared, birds ______.