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Natural Feature: Amazing Mysteries of Migration

Arkansas Natural Heritage Commission - Saturday, April 30, 2016

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.

What is Migration?

The word migration comes from the Latin migratus that means “to change.” At its most basic, migration is the movement of a group of animals from one place to another and, in most cases, back again. The majority of migration is seasonal, which is what we see when many birds return to Arkansas 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, called a partial migration. Red-tailed hawks are a good example of a bird that is a partial migrant. Mammals are 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. A good example of a nomadic migrant, the American bison once roamed in large herds 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, as evidenced by the ruby-throated hummingbird. It migrates from the Yucatan peninsula of Mexico to the southeastern United States every spring, a journey of 500-600 miles over the Caribbean Sea that takes 24 hours without a break.

Who Migrates?

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 and bats are examples of Arkansas mammals that migrate. Snakes migrate to den sites in the fall. Spotted frogs migrate to springs to lay eggs. 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!

How Do Migrants Find 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 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

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," and experiments with European starlings have shown them using the sun compass technique of navigation. Animals that migrate during the day are the most likely to use a sun compass to find their way.

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 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, allowing birds to find north. Many songbirds and shorebirds migrate at night using a star compass.

Magnetic Compass

The Earth’s 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 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

Scientists think that some animals use landscape maps when they migrate. Things like mountain ranges, rivers, or coastlines may serve as landmarks for navigation.

We do not yet understand all the mysteries of how migrating animals find their way. Some animals may use only one method while others may 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!

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