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Amazing Flight: Mysteries of Bird Migration

by Ann E. Kessen

In her 22 years as an avid birder, Ann Kessen has witnessed many of the wonders of bird migration. Here, she explores some of the reasons birds might undertake such dangerous journeys. Kessen is past president of the Minnesota Ornithologists’ Union, a Ph.D. student in the University’s Department of Ecology, Evolution, and Behavior, and an instructor in ornithology. She has worked with the Bell Museum’s bird collection since 1993.

If you’ve ever seen one at your backyard feeder, you will know that the ruby-throated hummingbird is tiny. An average adult weighs only one-tenth of an ounce. Though it is as light as a penny, this little bird makes a journey of 1500 miles each autumn, traveling from Minnesota and other parts of eastern North America to its wintering grounds in southern Mexico and Central America.

The hummingbird travels through unfamiliar territory, searching for nectar-rich blossoms and small insects while trying to avoid accidents, such as flying into windows. The exertions of the long flight, the unpredictable weather and the uncertain food supply may weaken it, making it more vulnerable to predators or disease. Its greatest challenge comes, however, with its arrival on the Gulf Coast. To complete its journey, it must fly across the Gulf of Mexico, approximately 600 miles. Since a hummingbird cannot land on water, the flight must be nonstop. With favorable weather, it takes about 18 hours for a hummingbird to cross that expanse of open water. If there is a head wind or a storm, or if the bird has not accumulated enough fat to power an 18-hour flight, then the hummingbird may perish in the waters of the Gulf. Many do make it, however, only to repeat this harrowing journey in the spring, traveling in the opposite direction.

The hummingbird is not alone in undertaking such a difficult migration. Many species of birds are known for their extraordinary migratory feats. The Arctic tern, for instance, breeds in the high Arctic and winters in the Antarctic. Its yearly migrations total nearly 22,000 miles, from one tip of the globe to the other. The blackpoll warbler, a half-ounce songbird, may fly nonstop for 85 hours as it crosses the western Atlantic from northeastern North America to northeastern South America.

These are dramatic examples of the demands of migration, but even for a species whose migration does not involve such long flights or require grueling marathons across open ocean, the journey through the hazards of unfamiliar territory and unpredictable conditions is demanding and dangerous. Each autumn in North America, approximately five billion land birds of about 200 species set out for Central and South America. Less than half return the following spring—not because they choose to go elsewhere, but because they didn’t survive the rigors of two migrations and the hazards of the wintering ground. Clearly, this whole migration business is risky. So why do birds do it?

This question can be answered on a couple of different levels. On an individual level, the bird at the feeder in your backyard flies south in the fall because it is genetically programmed to do so. For many species of birds, instructions for the direction and duration of migratory flight are written in their genes. In late summer and autumn in the northern hemisphere, when the shortening days reach a certain length, a physiological response to the amount of daylight will cause a bird to grow restless. This restlessness, called zugenruhe, is not random; it urges the bird toward a certain compass point, orienting it to the direction of its eventual migration. At this time, the bird will consume extremely large quantities of food in order to build up a store of body fat that will fuel its migratory flight. The actual start of migration will be determined by a combination of internal body rhythms and weather conditions, especially wind direction.

Beyond the mechanics of this extraordinary process lies a larger question: Why are birds genetically programmed to embark on such hazardous journeys? How and why did migration evolve? As we’ve already seen, migration is a costly behavior. It demands tremendous energy, presents numerous dangers and requires coping with uncertain weather conditions. When they finally reach the wintering ground, migrating birds must compete with local residents for food. Yet the prevalence of migratory behavior suggests that it must offer an advantage. It must permit an individual to leave more offspring over a lifetime.

How migration evolved is not known for sure, but it was probably a gradual process. According to most theories, some environmental pressure—for instance, a seasonal reduction in food supply—induced part of a population to move away, seeking better conditions. As these birds continued to respond to their environment, pressures in their new location may have prompted them to return to their original range for part of the year. Because they sought optimal living conditions throughout the year, the birds that relocated seasonally gained a greater chance to thrive. On the other hand, the part of the population that remained in one place gradually died out, over many generations, because constant environmental pressures limited their ability to produce offspring.

This hypothesis for how migration evolved makes sense. It does not, however, answer the question of why migration evolved. What was the pressure that caused birds to move temporarily to another place? Ornithologists have debated this question for many years. One widely held idea has been that glaciation explains the evolution of migration. Some believe that advancing glaciers pushed temperate zone birds into the tropics. When the glaciers retreated, the descendants of these birds returned to their ancestral homes. Others think that glaciers prevented tropical birds from spreading into temperate regions. When the glaciers retreated, these birds were able to spread into areas that had been ice-covered. Their descendants returned, however, to their ancestral homes in the tropics. One reason that these hypotheses are not widely accepted is that they don’t explain how the journey from the breeding grounds to the wintering grounds and back again became a repetitive behavior, enacted year after year.

Another hypothesis is that climate change may have sparked the evolution of migration. As the winters in temperate regions became more difficult, the reduced probability of surviving the winter caused birds to begin moving toward warmer climes. (Many birds tolerate cold very well; what they cannot tolerate is the negative effect of cold weather on their food supply.) In the spring, when the weather in temperate regions improved, migrants moved back again to avoid competition with tropical residents and to take advantage of the seasonal abundance of food. In addition, the long day length at higher latitudes allowed more time for foraging. A problem with this idea, and indeed, with all "northern ancestral home" hypotheses, is that many of our migrants, like the ruby-throated hummingbird, are known to be descended from tropical ancestors. Another possibility is that the ancestors of many migratory birds were tropical residents. Some of these birds dispersed into temperate regions to avoid competition for food and nesting territories. Higher latitudes offer an abundance of insects and more daylight in which to eat them. In addition, the number of breeding birds per acre is lower outside of the tropics. When nests are more spread out, offspring have a greater chance of survival because opportunistic nest predators are less likely to find—and pilfer from—widely spaced nests. Also, because competition for territory among male birds is not as great, inexperienced males have a greater chance of establishing territory, attracting mates, and reproducing.

In the winter, however, the food supply in temperate regions is reduced, so these birds moved back to the tropics until the weather moderated and the breeding season returned.

A problem with any of these hypotheses is that none of them seems to work for all species of migratory birds. Clearly, the "how" and "why" of the evolution of bird migration are questions without sure answers. Ornithologists continue to try to understand how this remarkable behavior came about, as well as to solve some of the other mysteries of migration. For instance, there are many unanswered questions about how birds navigate and about physiological adaptations for these long trips. There is also much to be learned about the ecology of migrants’ wintering grounds, an aspect of temperate breeders’ lives that has been viewed rather simplistically in the past. These examples illustrate only a few of the areas where additional research is needed. There is a great deal that we don’t understand about the remarkable annual travels of birds. I suspect, though, that no matter how much we learn, we will continue to be amazed by the tiny hummingbird’s extraordinary journey.