What external factors affect time of migration?
If pituitary functions and variations in day length were the only factors, migration would occur regularly every year. Such a lack of flexibility could be catastrophic for migrants because of variations in biological and meteorological conditions. Environmental factors such as weather, arrival of spring, flowering, foliation, insect hatching and availability of food vary from year to year. The pituitary prepares the bird for migration. The proper ecological conditions are necessary to trigger it. Birds can be exhausted and emaciated by the time they reach stopping areas. They gorge themselves to replenish their fat reserves before preparing for the next leg of the flight.
