Migration is the movement of an animal from one habitat to another. Birds migrate for a number of reasons and the timing of those migrations are one of the most fascinating phenomena in nature. Birds migrate in order to breed, find resources, and avoid cold weather. It can be triggered by a combination of changes from a lower temperature, day length, and genetic predisposition. Birds may migrate far distances, across states or short distances from higher to lower elevations. Some birds do not migrate and are able to find sufficient sources of food year around. Birds have many physical adaptations to protect them against cold weather. Their feathers provide great insulation. Birds also have oil that coats their feathers to provide waterproofing and warmth. Some grow additional feathers to give them thicker protection in the winter. Their body heat warms up the air between their feathers and fluffing produces air pockets that further insulates them.