For all birds, one of the driving forces behind migration is food scarcity. If all birds were to stay in the same tropical regions all the time, food would become less and breeding would be less successful. But as food sources regenerate in the north each spring, birds migrate to those areas to take advantage of the abundant supply. As the food supplies dwindle in the fall, they return to replenished tropical regions.
Over millennia, birds have evolved migration patterns, timing as well as destinations to disperse around the world to breed, taking god advantage of a wide variety of suitable conditions to raising their young.
Other reasons birds migrate include the climate, predators, and diseases in one area.