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.
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 be 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!
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!
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, disappears. 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.
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 habitat 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:
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.
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 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 from each other. This allows birds to find north. Many songbirds and shorebirds migrate at night using a star 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 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.
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.
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!
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 Whale||12,500 miles|
|Monarch Butterfly||2,000 miles|
|Chinook Salmon||900 miles|
|Arctic Tern||11,000 miles|
|Sooty Shearwater||20,000 miles|