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The Factors That Cause Migration

The majority of animals -- from butterflies to jellyfish to whales -- migrate. Some, such as the red crabs on Christmas Island, move only a few miles, while others, such as monarch butterflies and humpback whales, travel thousands of miles on their migratory treks. An individual creature might make the same trip many times over the course of its life, or it may make a single trip that takes most of its life span. A number of factors determine whether an animal or population migrates; but ultimately, because the Earth's resources in the animals' environment fluctuate, it no longer provides what the animals need.

Climate
- Changes in weather can factor into whether animals migrate. For instance, geese fly south from Canada and northern parts of the United States to spend the winter in warmer areas. They are just one example of animals migrating to take advantage of the change in seasons; other animals, such as sheep, may move to higher, cooler elevations in the summer and back down to more moderate areas in winter. Other animals might migrate based on rainy or dry seasons.
Food
- The availability of food can also drive an animal to migrate. The same geese that fly south in the winter because of the weather migrate back north because food supplies are more plentiful there. Gazelles and other grazing animals migrate across grassy areas, following the vegetation. Even tiny aphids migrate annually from the place that they spent the winter and between host plants in search of new sources of food. Animals might also migrate to a new area if their source of food is disrupted -- for instance, if another invasive species outcompetes them for their food supply.
Overcrowding
- Population is another factor that can result in migration. As a species' population in a given area increases, so does competition for things such as mates, food and shelter. In such cases, part of the population may benefit from migrating to an area where there is less competition for resources. In the 1800s, many people immigrated to the United States because resources were thought to be more plentiful -- and competition less tough -- than in their home countries. Bees and locusts are also examples of animals that migrate when their environment becomes overcrowded.
Breeding
- Sometimes animals migrate to an area so that they can reproduce or raise their young. Safety from predators may factor into the decision to migrate; the animals' original environment may have too many predators for them to raise their offspring safely. A species of animal that does not raise its young may migrate to a location that provides more shelter for the eggs it lays. Monarch butterflies migrate for a different reason -- they migrate north in the spring in search of emerging milkweed plants, upon which they lay their eggs.

Animals & Science