In a word, competition. Like every other species in the world, birds have to compete for the resources to eat, drink, and be reproductive. If they all decided to hang up their traveling lifestyles and retire to the tropics year-round, supplies would run out quickly and many species wouldn't be able to feed the next generation. Instead, evolution has given some birds the ability to fly across continents. It provides a less competitive way of life: They can raise their young in the verdant lands up north during the summer and pack up for the south when the thermometer drops and nesting season comes to an end.