Birds don’t migrate because of the cold. A little snow won’t actually kill them, but a scarce food supply will. Birds migrate to follow the bugs, worms or baby rabbits, but the “why” of migration isn’t nearly as fascinating as the “how”.
There are two skills that all migratory birds must possess: orientation and navigation. Orientation is the ability to determine the direction in which you’re travelling. Birds don’t have compasses or GPS devices, but they can orient themselves using the position of the Sun by day and the stars at night. Some birds, like pigeons, can orient themselves against the Earth’s magnetic field.
As for navigation, there are several theories, but no definitive answers. Some birds are believed to ‘pilot’ from one point to the next using large landmarks like coastlines, mountain ranges and even four-lane highways. Other birds learn the migration route by following older birds that have already made the trip. But the most remarkable birds are those who are born with a mental map imprinted into their DNA, able to make difficult long-distance journeys without any outside assistance.
Soaring birds like hawks and pelicans do most of their flying during the day to capitalise on rising thermal drafts. Smaller birds that rely on powered flight mostly fly at night when the atmosphere is more stable.