The main advantage of migration is easy enough to guess. During northern winters there's little food, and cold temperatures make life hard. Farther south, there's more food and less cold.

Birds do not migrate because they figure out that they need to. It's been shown that, at least in some birds, changes in day length cause glands in the birds' bodies to produce hormones that produce profound changes inside the birds, changes that prepare them for the flight south. In the fall, as days grow shorter, fat accumulates under the skin. This fat contains energy needed for those coming days when the birds will be spending more energy flying than they'll be eating during their occasional rests. Weather changes sometimes trigger a migration's start, but by then the birds are already prepared. The urge to migrate must be very powerful. Starlings caged as their migration time approaches become extremely fidgety and point their bodies in the direction they want to fly toward.