Animals are found in all climates from the coldest to the hottest. They can be observed from very high altitudes to many kilometers below the sea. If a certain zone is studied for a whole year, it is possible to notice that the animals which inhabit that zone change from season to season. Certain types of birds, for example, seem to disappear in the cold months of winter only to reappear in the spring. Like plant populations, animal populations vary greatly in the course of a year.

In order to remain active, animals need a good food supply throughout the winter. Large herbivores, such as deer, do not get cold very quickly and they can usually find enough food during the winter months. Instead of eating grass and herbs which may be covered in deep snow, they may have to eat the bark of trees and the branches and shoots of bushes.

Carnivorous mammals and birds may bunt for herbivores which are still active. For example, foxes can still find mice, voles or rabbits to eat in winter. If, however, a carnivore specializes in insects (an insectivore) it may find no food at all in the winter months.

Animals which are not adapted to cold weather, however, need to find other solutions to the problems of survival. There are two main techniques of survival : migration and hibernation.

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