Behavioral Cycles: Behavioral Cycles What is Migration?: What is Migration? Migration is a movement of animals from one region to another for survival. Animals may migrate in order to find food and other resources, to stay warm, or to mate or give birth. Most animals migrate seasonally. For many, this means they migrate in one direction during fall and then back again during spring. How Do Animals Know When to Migrate?: How Do Animals Know When to Migrate? Migration is a behavioral adaptation that is instinctive in the animals that do it. Like hibernation and dormancy, it is a behavioral cycle. Various cues from the environment trigger migration in animals, cues such as changes in daylength and temperature, as well as a dwindling food supply. Hormonal cues also seem to play a role, particularly in those animals that migrate in order to breed, but in some others as well. How Do Animals Know Where to Go?: How Do Animals Know Where to Go? There is evidence to suggest that animals use one or more of the following cues to help them figure out where to go: - the sun - star patterns - Earth's magnetic field - smell - geographical landmarks such as mountains and coasts - weather patterns We'll talk more about how various animals use these factors as we examine the migratory behavior of several animals. Arctic Tern Migration: Arctic Tern Migration Even if you know next to nothing about migration, you're probably well aware that many birds migrate. Some birds migrate relatively short distances. The North American blue grouse, for instance, travels about 300 miles each year on its trip from deciduous woodland to pine woodland in winter and then back again in summer. Other birds migrate astonishing distances. Take the Arctic tern, for example, which flies a total of about 22,000 miles each year on its trip from the Arctic to Antarctica and back. Because each of these journeys lasts about three months, Arctic terns spend half of each year migrating. The animation below shows just one of several migration routes used by Arctic terns. Arctic Tern Migration (cont.): Arctic Tern Migration (cont.) If you said that Arctic terns require weather that is fairly cold, but not too cold, you are correct! Arctic terns are summer seekers; albeit they seek summers in the coldest places on Earth. They give birth in the Arctic during the Northern Hemisphere's summer. Then they head south to the Antarctic in time to enjoy the Southern Hemisphere's summer. There they feed and relax after a long journey, not to mention prepare for the next journey. To prepare for migration, birds store up extra fat to help give them a good supply of energy, particularly for when they have to cross areas where food resources are scarce. Some birds go so far as to double their body weight! Arctic Tern Migration (cont.): Arctic Tern Migration (cont.) One very strong indicator that it's time to migrate has to do with the amount of sunlight. Think about the positions of Earth's poles relative to Earth's revolution around the sun. During summer in the Arctic, the days and nights both are bathed in sunlight because the North Pole leans toward the sun. That's why the Arctic is known as the land of the midnight sun. However, as summer comes to an end, the sky begins to dim. Eventually, by winter, the sky will be dark day and night. Thus, as the light begins to dim in the Arctic sky, Arctic terns know it's time to get going. The same goes for Antarctica, which is lit 24 hours a day in summer too. As the light begins to dim in March, it's once again time for Arctic terns to migrate. Of course, other changes may come into play too. When the Arctic's 24-hour sunlight begins to dim, the temperature begins to drop. Arctic Tern Migration (cont.): Arctic Tern Migration (cont.) You should notice from looking at the Arctic tern's migration route that it rather closely follows the coasts of the continents, suggesting the bird uses these geographical markers as navigation cues. The Arctic tern is also known to use the sun and stars as navigation cues. Both work as compasses. Bird Migration Cues: Bird Migration Cues Many other migrating birds use these same cues. We know birds rely on geographical cues because particular routes are popular. For instance, in the United States there are four main bird migration routes: the Atlantic, Mississippi, Central and Pacific flyways. These routes are marked by mountain ranges, coastlines, and rivers that run north to south. Many birds migrate at night. This is probably because many of them are daytime feeders. We know that they use the stars as a navigation tool because scientists have observed that many birds will not migrate when the night sky is especially cloudy. Birds rely on weather patterns as navigation cues too. When they fly south for winter, for example, many birds wait for wind patterns to shift so that wind is blowing from north to south; the wind helps carry them along. In the spring, it's just the opposite; the wind reverses direction, blowing from south to north, and birds wait for this shift. Scientists have also discovered evidence that birds use Earth's magnetic field to help them navigate. Frog Migration: Frog Migration While the fact that birds migrate is common knowledge, that frogs, as well as other amphibians such as toads and salamanders, migrate, may be less common knowledge. Each spring, amphibians emerge from their hibernation spots in order to migrate. Although where various species hibernate differs, they all choose moist places. Some hibernate in mud at the bottom of lakes, others under fallen leaves. They migrate to nearby ponds or vernal pools to mate and lay their eggs. They travel at night, typically when the weather is rainy or very humid because their skin must stay moist in order for them to survive. In fall, they migrate back to their hibernating spots. Whale Migration: Whale Migration Among animals that migrate is also the gray whale, which spends its summers in the Bering and Chukchi Seas in the Northern Pacific and Arctic Oceans. In fall, the whales migrate south to Baja California, Mexico, where they mate and give birth to their calves. In late winter or spring, they migrate back north. It is believed that the whales eat little food, if any, while in their breeding grounds. They survive off of the many tons of blubber stored up during their time in their summer feeding grounds up north, and when they return north, they replace the fat they lost during their migration. Whale Migration (cont.): Whale Migration (cont.) Based on the migration route shown on the previous slide, one can see that, like Arctic terns, gray whales use geographical markers such as coastlines to help them navigate. They also use Earth's magnetic field to help them navigate. Like many other animals, gray whales have magnetite in their retinas. This magnetite helps them sense Earth's magnetic field. Salmon Migration: Salmon Migration Adult salmon migrate from their ocean homes to the rivers and streams where they were born in order to produce their own offspring. Because they must swim upstream, the journey takes a lot out of them. They don't eat at all along the trip. Salmon are an example of an animal that uses smell to navigate during their migration. Their sense of smell helps them locate the very rivers and streams where they were born. Migration Hazards: Migration Hazards Now that you've learned about the migratory behavior of several animals, let's now think about the hazards involved in migration. There are two types of hazards migrating animals may encounter: natural and human-caused. Natural hazards include the following: severe weather such as storms and droughts food scarcity predators (for instance, migration may make animals more visible and more vulnerable to predators) physical demands Migration Hazards (cont.): Migration Hazards (cont.) Human-caused hazards may include the following: human predation such as hunting habitat destruction migration route obstructions such as fences, roads, skyscrapers, radio towers, and dams Each year millions of migratory birds are killed by skyscrapers, radio towers, and the like. Some crash into windows that reflect the night sky. Others are confused by lights on these tall buildings and may fly in circles until they die of exhaustion. Many birds' lives can be saved by switching off the lights on skyscrapers and such. Dams may block fish from swimming upstream to spawn. However, some dams include structures called fish ladders that help fish continue on with their migrations.