Fall Migration

Fall is upon us, can you feel it? The sun is lower in the sky, leaves are starting to change color and fall migration has begun.

Why do animals migrate? This movement exists in all the main animal groups, which includes birds, fish, insects, amphibians, crustaceans, mammals, and reptiles and refers to the movement of these creatures over a long distance, usually in line with changes in the seasons.

Not all migrations are the same. Grey Whales migrate between the warm waters of Mexico to the cold Arctic seas, while brown bats migrate only a very short distance. Certain animals take breaks along the way, while others travel nonstop. Hummingbirds bulk up before their big trek, or they may stop and eat along the way. Animals also differ greatly in the ways that they are able to navigate. Some animals, like homing pigeons, use their sense of smell, while others follow trails, use the Sun and stars, or follow coastlines. Yet others, like the arctic tern, feel the Earth’s magnetic pull. Many animals know where to go instinctively, while others (like Canada geese) have to be taught by their parents. There are many reasons why animals may determine that it’s time to migrate; they may be prompted by a change in temperature, in the length of daylight, or even in hormones that cause them to eat more and save fat for the journey. The primary motivation for migration in the fall is the availability of food, even though there may be food still available, the triggers mentioned above will prompt the animals to start their journey.

There are a number of different types of migration.

Complete migration: All members of the population migrate. An example would be caribou in Alaska

Partial migration: Some animals migrate, some do not. An example would be the American Robin

Differential migration: Migration varies by class (e.g. age or sex) such as in the herring gull where young birds migrate a shorter distance than older birds and in the eastern United States female Dark-eyed Juncos migrate farther into the winter range than males.

Interruptive migration: Animals migrate some years but not others. Blue Jays exhibit this behavior as do some winter finches and snowy owls.

One of the most impressive feats of migration is that of the monarch butterfly. The monarch is the only butterfly known to make a two-way migration as birds do. Unlike other butterflies that can overwinter as larvae, pupae, or even as adults in some species, monarchs cannot survive the cold winters of northern climates. Using environmental cues, the monarchs know when it is time to travel south for the winter. Monarchs use a combination of air currents and thermals to travel long distances. Some fly as far as 3,000 miles to reach their winter home! The eastern population of North America’s monarchs overwinters in the same 11 to 12 mountain areas in the States of Mexico and Michoacan from October to late March. Monarchs roost for the winter in oyamel fir forests at an elevation of 2,400 to 3,600 meters (nearly 2 miles above sea level). The mountain hillsides of oyamel forest provide an ideal microclimate for the butterflies. Here temperatures range from 0 to 15 degrees Celsius. If the temperature is lower, the monarchs will be forced to use their fat reserves. The humidity in the oyamel forest assures the monarchs won’t dry out, allowing them to conserve their energy. Fortunately for the butterflies, the Mexican government has created the Monarch Butterfly Biosphere Reserve to protect their winter habitat.


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