Actually they come back to breed. Heat is the enemy of baby birds. That's why birds come back to a cooler climate in the spring. Also, protein sources are abundant in spring. All those little insects that the parents catch help their baby birds to grow fast and strong. Once the breeding season is over and all the little ones have matured enough to follow their parents, off they go to enjoy the food and warmth of their winter homes. Northern Cardinals can have up to 4 families during the breeding season!