Animal Migration

Animal Migration Facts

Animal Migration ['ăn-ə-məl] [mī-'grā-shŭn]

The movement of an animal from one region or habitat to another.

Going Places — Migration

It's March and spring is on its way. What signs of spring do you look for — flowers, leaves on the trees, or warmer temperatures? For many people spring begins when birds return, filling neighborhoods and parks with their songs. The annual migration of birds is one of the world's great wildlife spectacles. But migration is not just for the birds! As it turns out, many kinds of animals migrate. Let's take a closer look.

flock of migrating birds

What is migration?

At its most basic, migration is the movement of a group of animals from one place to another and, in most cases, back again. Most migration is seasonal. That is what we see when many birds return to Idaho in the spring and leave in the fall. These birds also represent a complete migration because all members of a species leave. Sometimes not all members of a migratory population leave an area. This is called a partial migration. Red-tailed hawks are a good example of a kind of bird that is a partial migrant.

Mammals are often a good example of a group of animals that are considered nomadic migrants. They wander from place to place, usually with the change of seasons. They might end up where they started or they could go somewhere completely different. The American Bison is a good example of a nomadic migrant. Great herds of bison once roamed throughout the Great Plains as the seasons changed.

Not all migration journeys are long. While some birds fly incredible distances when they migrate, some amphibians may only move a quarter of a mile when they migrate. Size does not seem to matter when it comes to migration distances. Idaho's Black-chinned hummingbirds leave Idaho and fly all the way to Central America where they spend the winter. Then they fly all the way back to spend the summer in your backyard!

Who Migrates?

Monarch Butterflies on tree branch

When you think about migration, you probably think of birds. That is because birds are all around us and we notice when they leave and return. We hear geese overhead in the fall and notice when the hummingbirds stop coming to our feeders. Do you notice when the monarch butterflies migrate? How about bats? What about the frogs that suddenly showed up at a pond? These are all examples of migratory animals.

Birds, mammals, reptiles, amphibians, fish, and even insects migrate. Deer, elk, moose, bighorn sheep, and pronghorn are examples of Idaho mammals that migrate. Gopher snakes, rattlesnakes, and other snakes migrate to den sites in the fall. Spotted frogs migrate to springs to lay eggs. Salmon migrate from Idaho to the Pacific Ocean and back to spawn. Monarch butterflies fly south to winter in Mexico, and some species of dragonfly migrate along the Pacific coast in the fall. So migration is really not just for the birds!

Why Migrate?

Most animals that migrate live in places that have definite seasons. Take Idaho's summer bird residents, for example. Idaho's spring, summer, and early fall provide these birds with plenty of good habitat. Once winter comes, the habitat changes. The insects these birds eat disappear. Water and shelter become harder to find. If these birds are to survive, they have to leave to find the food, water, and shelter they need.

herd of deer

As it turns out, food is the single most important reason that animals migrate. Idaho's deer and elk migrate to find food. During the summer months, these large mammals can be found up in the mountains feeding on grasses and other plants. As winter brings snow to the mountains, deer and elk move out of the mountains, into valleys where the snow is not as deep and they can find food. This is an example of elevational migration.

Animals also migrate to avoid extreme heat or cold. Such extreme climates often also impact the availability of food. Animals leaving these harsh environments can avoid the heat or cold and find the food they need.

Animals also migrate to find good habitats to raise their young. Idaho's spring and summer provide a lot of food for insect-eating birds to feed their young. Adult salmon make a 900-mile trip from the Pacific Ocean to the rivers and streams of Idaho to lay their eggs. Young salmon can find the aquatic insects they need to survive and grow before they make their own migration to the ocean.

Finding Their Way

Exactly how animals migrate has been one of the great mysteries of science. Before people even knew about migration, they had some very unusual explanations for the seasonal movement of birds. Some thought that birds spent the winter under the mud of lakes. Other ancient scientists thought that one kind of bird turned into a different kind of bird for the winter!

Today we know these explanations are not even close to being correct, but migration is still a bit of a mystery. Scientists are much closer to understanding this mystery. They have studied many animals, especially birds, and done many experiments to see if they can discover exactly how animals migrate. Here are some of their ideas:

Sun Compass

bird flying sun compass

Some migrating animals may use the movement of the sun across the sky to find their way. Since the sun changes position as the Earth rotates, these animals would need to be able to make adjustments to their path of travel so it is not affected by the sun's movements. This is called "time compensation." Experiments with European Starlings showed that this is what happens. Animals that migrate during the day are the most likely to use a sun compass to find their way.

Star Compass

star compass

The star compass is kind of like a nighttime version of the sun compass. So far, it has only been found in birds. Scientists have discovered that young birds learn the position of the north by observing the pattern of stars surrounding the North Star, Polaris. These include some familiar constellations such as the Big Dipper, Little Dipper, Draco, Cepheus, and Cassiopeia. While these constellations rotate around the North Star, they stay in the same position in relation to each other. This allows birds to find north. Many songbirds and shorebirds migrate at night using a star compass.

Magnetic Compass

magnetic compass

Earth has two magnetic poles, the South Pole and the North Pole. These two poles cause the Earth to act like a really big magnet. Going north and south between the two poles are invisible magnetic lines of force. They make up the Earth's magnetic field which is what makes your compass work. The magnetic field is stronger at the poles and weaker at the magnetic equator which is a bit different from the equator drawn on a map or globe. At some points, the magnetic field touches the earth at an angle called the dip angle. Birds and other animals such as sea turtles can find north and south because they are able to detect the magnetic lines of force. While they are not sure how scientists believe that birds can also detect the dip angles. This would help them know how far to the north or south they have moved.

Polarized Light

polarized light

Polarized light comes from special kinds of light waves and it comes in many forms. It creates a pattern in the sky that stays the same as the sun moves across the sky. Even if the sky is cloudy, animals can still tell the position of the sun based on the pattern of polarized light. This kind of navigation system is used by some insects, amphibians, fish, and birds.

Landscape Maps

Think about how you navigate around your neighborhood. What about the route you walk to school or ride your bike to a friend's house? You use clues in the landscape around you to find your way. Maybe you turn right by the big hill or go left by the big pine tree. These are examples of landscape clues you use to navigate. Scientists think that some animals use landscape maps when they migrate. Things like mountain ranges, rivers, or coastlines are all possible ways of navigating.

landscape map

We do not yet understand all the mysteries of how migrating animals find their way. Some animals may use only one method while others use a combination. It is possible that a completely new way of navigating will be discovered. The one sure thing is that wherever they go, these amazing migratory animals know exactly how to get there!

Long-distance Travelers

grey whale
Grey whale

Migration is an amazing journey. The thought of tiny, one-ounce birds flying thousands of miles is truly mind-boggling. But even among big-time travelers, some stand out. Here is the one-way mileage of some migratory distance champions:

Gray Whale12,500 miles
Caribou700 miles
Monarch Butterfly2,000 miles
Chinook Salmon900 miles
Arctic Tern11,000 miles
Sooty Shearwater20,000 miles

Top 10 Questions

Thanks to Jeff Knetter, upland game and waterfowl staff biologist, Idaho Department of Fish and Game; and Vicky Runnoe, conservation educator, Idaho Department of Fish and Game for the answers.

  1. Why does migration happen?

    Migration is the movement from a place an animal lives to a place that provides something that it needs. So, if a food source disappears because of winter weather, an animal may leave to find food someplace else. They can come back. Food is the main reason for migration. (From Joe in Mrs. Schweitzer's class at Riverside Elementary School in Boise.)

  2. If mammals are warm, why do they migrate?

    Many animals migrate for food, not warmth. Mule deer or elk, which may summer at a higher elevation, will move to a lower elevation or a different aspect of slope so they can access vegetation more easily. (From Maddie in Jan McCamish-Cameron's class at Grace Jordan Elementary School in Boise.)

  3. How do animals know when to migrate?

    Animals and humans have a special gland in their brains that tells them when the light is changing. As the days get shorter and there is less light in winter, animals sense this and get what is called migratory restlessness. It lets them know, gosh, it's time to get out of here. (From Luke in Mrs. Schweitzer's class at Riverside Elementary School in Boise.)

  4. How long does it take to migrate?

    It all depends on the species and the type of winter. If it's a mild winter, animals like deer or elk will not have to travel as far as they do when it's a more severe winter. Steelhead and Chinook salmon take months to cover the ground they have to cover. In addition, some birds take weeks to migrate and some only take a few days. (From Joe in Mrs. Woodall's class at Dalton Elementary School in Coeur d'Alene.)

  5. Do whales migrate?

    They do. They will move from summering grounds, where there's a lot of krill for them to eat, to other areas where they can find their food. In February, you can find the humpback whale in Hawaii. Then it heads north to the Arctic for the summer where there is a lot of food for it to eat. Even a big animal living in the ocean will migrate to find the food it needs. (From Morgan in Mrs. Hunt's class at Cynthia Mann Elementary School in Boise.)

  6. What do animals do when they are migrating?

    They are moving for the most part. Birds and mammals move across the landscape, taking time to rest. They also need to eat and drink water to make sure they have the energy to reach their destination. (From Gabe in Jan McCamish-Cameron's class at Grace Jordan Elementary School in Boise.)

  7. Why do birds fly south? Do they fly other ways?

    They do. Some birds do not have to fly south to get to the place where they find food. The Northern Wheatear is one example of this where they nest in Alaska and fly south and west to end up in Africa. Birds will fly in a direction that's going to take them to the habitat they need for the winter and then they head back for the summer. (From Gurum in Jan McCamish-Cameron's class at Grace Jordan Elementary School in Boise.)

  8. How far can a bird fly without stopping?

    Some birds, like the Ruby Throated Hummingbird, will fly all the way across the Gulf of Mexico without stopping. Birds have been recorded flying two full days, or 48 hours, before they'll touch down. They need to have a lot of energy to make these long trips! (From Bridgette in Mrs. Hudson's class at Dalton Elementary School in Coeur d'Alene.)

  9. Do animals, like fish and caribou, travel together to migrate?

    I'm not aware of any that do other than waterfowl, which migrate together (like ducks and geese). Then there are species that travel in herds, like deer and elk. (From Alexa in Mrs. Adcock's class at Hillview Elementary School in Ammon.)

  10. Animals don't have GPS, so how do they know where they are going?

    Animals may use sun or light to help them migrate. Also landmarks can help them, like rivers or forests. If they are traveling at night, they may be guided by the stars or by the pull of the magnetic poles on the earth. Animals use many things to help them know where they are and where they are going. (From Cassie in Mrs. Woodall's class at Dalton Elementary School in Coeur d'Alene)