Winter range is an area that animals use in winter for food and cover. Generally, winter range contains a food source and thermal cover that together maintain the organism's energy balance through the winter, as well as some type of protective cover from predators. Although some species of animals have special adaptations, such as hibernation, to survive winter climates, many must migrate from their summer ranges when conditions there become too harsh. Elk (Cervus elaphus) inhabiting mountainous regions, for example, often move from higher ground to lower in the fall, avoiding the early snow cover at higher elevations. Nothern populations of caribou or reindeer (Rangifer tarandus) often travel over 600 mi (965 km) between their summer ranges on the tundra and their winter ranges in northern woodlands. Still more extreme, the summer and winter ranges of some animals are located on different continents. North American birds known as neotropical migrants (including many species of songbirds) simply fly to Central or South America in the fall, inhabiting winter ranges many thousands of miles from their summer breeding grounds.