Migration

Migration is a fascinating aspect of animal ecology. Migration inspires us whether we are studying salmon migrating thousands of miles back to their spawning grounds, huge flocks of sandhill cranes migrating across the northern skies, or caribou crossing rivers in the fall.

Migration has captured the interest of humans for centuries. Ancient civilizations devised many myths to explain the periodic appearance and disappearance of vast numbers of animals. For instance, people once thought that tiny birds called swallows buried themselves in the mud at the bottom of lakes to get through the winter. Instead, scientists found out that swallows fly all the way from Europe to Africa and back in one year. Perhaps the truth was harder to believe than the myth.

What is migration and why do it?

Animals that live in habitats that are difficult to survive in year round, must evolve a way to cope with the difficult time of year. A strategy used by many mammals and other species is hibernation. Migration is another option for animals that can move across long distances. They survive by leaving the area for part of the year or part of their life, and move to habitats that are more hospitable.

The most common reason to migrate is to take advantage of food, shelter, and water that vary with seasons, or life stage. The availability of food and water can change throughout the year. For instance, the lack of insects and leaves in the winter means there is less food to eat. Some environments have a rainy and a dry season that are very different. Temperatures change between the seasons, some areas getting very cold or very hot which can be hard on some species. Sometimes it is not about getting food but about staying safe. Deep snow may make animals easier to catch by predators, or animals may go to special breeding grounds to keep their young safe when they are especially vulnerable.

Many birds migrate in the fall. Because the trip can be dangerous, some travel in large flocks. For example, geese fly in noisy, "V"-shaped groups. Other kinds of birds fly alone.

How do they know when it is time to leave for the winter? Scientists are still studying this. Many see migration as part of a yearly cycle of changes a bird goes through. The cycle is controlled by changes in the amount of daylight and the weather.

Birds can fly very long distances. For example, the Arctic tern nests close to the North Pole in the summer. In autumn, it flys south all the way to Antarctica. Each spring it returns north again.

Most birds migrate shorter distances. But how do they find their way to the same place each year? Birds seem to navigate like sailors once did, using the sun, moon and stars for direction. They also seem to have a compass in their brain for using the Earth's magnetic field.

Other animals migrate, too. There are a few mammals, like some bats, caribou and elk, and whales that travel in search of food each winter. Many fish migrate. They may swim south, or move into deeper, warmer water.

Insects also migrate. Some butterflies and moths fly very long distances. For example, Monarch butterflies spend the summer in Canada and the Northern U.S. They migrate as far south as Mexico for the winter. Most migrating insects go much shorter distances. Many, like termites and Japanese beetles, move downward into the soil. Earthworms also move down, some as far as six feet below the surface.