The Whys and Hows of Bird Migration
As spring moves across the land, it triggers a fantastic movement of animals. Thousands of birds, for instance, the snow geese and sandhill cranes at Bosque del Apache in New Mexico, leave their winter homes and travel north to Canada and the Arctic. Valley-dwelling deer and ladybugs return to mountains finally free of winter snow. Songbirds and monarch butterflies return to the desert from Central America.
This seasonal migration is one of the most fascinating feats of the animal world. Although birds are often the most evident of the migrants, animals as small as pinhead-sized spiders and as large as blue whales migrate by land, air or water. Movements can range in length from less than a mile, for Eurasian milkweed bugs, for example, to the incredible 25,000 miles per year of the arctic tern. Migrations may only be seasonal or may take a lifetime to complete.
Migration can be defined as cyclical change of habitat oriented toward exploiting optimal environmental conditions. Basically, animals move between two locations to take advantage of better seasonal habitats.
Humans have pondered the whys and hows of migration for thousands of years. Early observers understood little about the seasonal movement of animals. Many people believed, for instance, that swallows hibernated in mud at the bottom of lakes. One "Person of learning," as he called himself, even wrote that animals overwintered on the moon. Modern scientists now have a much better understanding of migration, although many questions of how animals navigate still remain unresolved.
Three different, but somewhat interrelated, "whys" force animals to leave one prime habitat for another.
An area may be too hot or too cold during part of the year. Most birds that breed in the north cannot withstand the cold of winter, so they move south for warmer weather. For example, grassland sparrows move south from the Great Plains to the desert grasslands because snow covers their northern homes. Rough-legged hawks and merlins abandon their northern homes in winter. In contrast, some animals cannot endure the heat of summer, and they escape by migrating a handful of miles out of a valley and up into the mountains. Mule deer in the American southwest seasonally complete this cycle between mountain and desert, as do juncoes, white-crowned sparrows and goshawks.
Food is often available only seasonally, forcing animals to migrate to find optimal resources. Birds that overwinter in the tropics, especially insect feeders, abandon the north when insect populations drop. Lesser long-nosed, Mexican long-tongued and Mexican free-tailed bats all overwinter in Mexico and migrate north in summer to take advantage of flowering saguaros and organ pipe cactus. One of the best known migrants is the monarch butterflies, which move north in search of milkweed plants.
Some animals require a specific environment for laying eggs or giving birth to young. Green turtles only lay their eggs at the beach where they themselves were born, while salmon only deposit eggs in their birth stream. These migrations may last as little as year or may take many years to complete.
The "how" side of the question is harder to answer. Animals use olfactory, celestial and magnetic factors to navigate during migration. They can employ one or more of these facilities to determine north and also to create a navigational map, which is analogous to being able to determine latitude and longitude. Researchers are still trying to determine the full story of how the information is used by animals.
Salmon are probably the best known odor-based migrant. Studies indicate that young salmon are somehow "imprinted" with the odor of their birth stream. During their lives, salmon may travel thousands of miles from their home stream to the ocean and back again. When they return to breed and die, they sniff their way home by detecting chemical clues in the water.
Like humans, some animals navigate by the sun and stars. Researchers have shown that animals as diverse as fish, bees and birds derive compass directions from the sun. Birds even wander randomly, getting "lost" on overcast days, only to quickly reorient themselves when the sun appears again. Night time migrants on the other hand, find a compass direction by monitoring the stars. Again, overcast skies can lead to migratory misdirection.
The third means of animal navigation is the most recently discovered. Over the last few decades, scientists have discovered that numerous animals contain microscopic crystals of magnetite, an iron-based mineral, within their bodies. These animals use magnetite to detect subtle changes in the total intensity and/or inclination of the earth’s magnetic field, which allows the animal to determine direction and to form a navigational map. Magnetite occurs in creatures as diverse as monarch butterflies, bats, birds and salmon.
Migration also raises issues of conservation. We often like to think of birds that we see in our own region as "our" birds and that they only migrate south for the winter. If we consider the fact that these birds may spend eight months in the south and only four months in the north, then we might think of them as southern species who only visit us during favorable conditions. As such, we should be concerned with conservation in these bird’s true home territory and along the route they travel during migration. Migration makes us aware that our planet is a relatively small place of interconnected ecosystems.
Editor’s Note: As an FOB (Friend of Bugs), freelance writer David B. Williams, author of A Naturalist’s Guide to Canyon Country.
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