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Habitat Considerations
Habitat considerations are one of the primary reasons for the migration of animals throughout the world. At the most basic level, habitat considerations entail the availability of essential resources like water, food and shelter. Food resources like plant and insect populations fluctuate with the seasons and affect migration. The same goes for fresh water supply. The overcrowding of a species in an area also impact migration patterns. Norway lemmings and desert locusts migrate when overcrowding causes food scarcity and competition for resources.
Weather
Many animals use migration as a substitute for hibernation and a means of combating inclement weather. Cold winters and heavy snowfall make life difficult for small animals, particularly those preyed on by larger animals. Monarch butterflies exhibit this migratory pattern. Some animals, such as gazelles and zebras, migrate toward areas experiencing heavy rainfall. They do this because they feed on the plants that flourish in the aftermath of large storms.
Breeding
Many animals migrate for purposes related to breeding, particularly aquatic or semi-aquatic animals with very specific environmental considerations for procreation. Humpback whales, for instance, migrate from the food-rich waters off the coast of Alaska to the warm waters of the Hawaiian Islands in order to give birth. The sperm whale also migrates to mate and give birth. Sea turtles migrate each year to the beach on which they were born to lay eggs, while frogs and toads use specific ponds for egg laying and migrate to these ponds annually.
Other Reasons
A number of other reasons for migration exist. African elephants and wildebeests migrate in search of certain minerals essential to their diets. The walrus and yellow-lipped sea krait, a species of sea snake, both migrate to safe places while they molt, as molting leaves them vulnerable to attack. Great gray owls and northern finches exhibit irruptive migration, meaning they migrate at random times of year to random places for reasons indiscernible to scientists. Little brown bats migrate in search of an ideal cave for hibernation.