Every fall, several billion North American birds leave their breeding grounds and migrate to other areas to spend the winter. But why do birds migrate? In order to understand better, let’s look at the risks, benefits, and natural evolution of this annual event.
The Risks of Migration
Every fall, several billion North American birds leave their breeding grounds and migrate to other areas to spend the winter. Some will go only as far as the next state while others head all the way to South America. The tundra-breeding arctic tern will travel up to 25,000 miles to get from its summer home in Alaska and Arctic Canada to its wintering grounds off the coast of Antarctica—the longest migration of any bird species in the world.
Birds that migrate take on considerable effort and risk. They burn large amounts of energy and expose themselves to new predators, diseases, and hazards while passing through unfamiliar territory. In the modern world, there are also human-caused dangers that birds never evolved to cope with.
Migrating birds can be confused and led astray by artificial lights or tricked into landing on wet asphalt that looks like open water. Birds in migration are also especially vulnerable to window strikes and collisions with other structures like wind turbines. In 2009, National Geographic reported that migration-related mortality is responsible for up to 85% of all songbird deaths.
Learn more: Essentials of Bird Migration
So that raises an important question. If migration is so dangerous, why has evolution crafted the migratory instinct in so many species? The short answer is that it beats the alternative. Many North American birders think of our forests and grasslands as “home” to all the birds that breed here and imagine that migrants are going on a winter vacation to the tropics, but it might be more accurate to think of it the other way around. For migrants, their breeding territory is a nice place to visit but they wouldn’t want to live there through the harsh winters.
The Benefits of Migration
Without migration, many insect-eating species such as most warblers, tanagers, and flycatchers would be confined to only the warmest parts of North America. They would miss out on the bonanza of food and breeding opportunities available further north in the summer. The same goes for waterfowl, which would not be able to occupy any area where water freezes over in the winter if they couldn’t migrate back to warmer climes come fall. In general, a species of bird that migrates does so because there are advantages to breeding in the summer in territory that would be inhospitable in the winter.
But then why migrate north at all during the summer you might ask? The answer lies in the predictability of favorable conditions during summer in the temperate zone of North America. In the Neotropics, although there are fruiting trees and insects year-round, there is also a high density of resident birds competing for the same food source and a higher density of potential predators.
In North America, during summer there is a very high insect load and fruiting and flowering trees with comparatively lower densities of birds and predators. And also there is the advantage of longer days to take advantage of this abundance of food. These conditions are perfectly suited to raising a brood of hungry offspring.
Evolution and Migration
One theory on the evolutionary origin of long-range migration is that it dates to the end of the last Ice Age. As the glaciers retreated, tropical birds ventured farther north for the longer days and a seasonal increase of insects and other foods as we just mentioned. Over time, the distances involved became longer and longer, stretching to hundreds or thousands of miles. Some species of resident birds decided to remain year-round while others evolved to migrate, creating a balance and reducing competition between bird species.
In general, many migratory species are flexible and continually adapting to new conditions. Over recent decades, changes like milder winters and a rise in the number of bird feeders have allowed some migratory species to winter further north than they used to.Despite this flexibility, long-distance migration involves complex routes and patterns that have developed over thousands of years. To get the most out of migration as a birder, you must understand the factors that influence the timing of migration and where migratory birds stop along their journeys.
Changes in the food supply, weather, and day length can trigger the migratory impulse in some species. Other species appear to be genetically “set” to migrate at certain times, with a powerful travel instinct that can appear even in captive individuals who are well-fed and protected from the weather. Overall, the mechanisms that determine the timing of migration are still imperfectly understood by science.
Whatever gets them started, once they are underway, many long-distance migrants follow predetermined routes that their ancestors have traveled for thousands of years. Without the benefit of Google maps or GPS, even first-year birds of many species can make it to their destination flying on their own. In fact, in some species migration is rigidly segregated by age.
Immature osprey typically migrate later than their parents and follow different routes, sometimes taking detours (which may help them discover potential new territories) or making epic over-water journeys that the more experienced adults prefer to avoid. Other birds, like snow geese, migrate in family groups so that older birds can help the youngsters avoid potentially fatal mistakes.