When many of us think about migration, the image of geese winging their way south in their wrinkled V-shaped flocks is one that often comes to mind. The migration of geese is an example of the annual, large-scale movement of birds between their breeding (summer) homes and their nonbreeding (winter) grounds.
More than 650 species of birds nest in North America. Some are permanent residents and live in the same area year-round. The majority of the species, however, are migratory.
Why do birds migrate?
Birds migrate to move from areas of low or decreasing resources to areas of high or increasing resources. The two primary resources being sought are food and nesting locations.
Birds that nest in the northern hemisphere tend to migrate northward in the spring to take advantage of burgeoning insect populations, budding plants and an abundance of nesting locations. As winter approaches, and the availability of insects and other food resources drops, the birds move south again.
Escaping the cold is a motivating factor but many species, including hummingbirds, can withstand freezing temperatures as long as an adequate supply of food is available.
Types of migration
The term migration is used to describe movements of populations of birds (or other animals). One way to look at migration is to consider the distances traveled.
- Short-distance migrants: May move only a short distance, as from higher to lower elevations on a mountainside.
- Medium-distance migrants: Some species may cover distances that span from one to several states.
- Long-distance migrants: Birds that typically have ranges that extend from the United States and Canada in the summer to Mexico and further south in the winter.
The pattern of migration can vary within each category, but is most variable in short and medium distance migrants.
Origins of migration
The origin of migration is related to the distance traveled. For short-distance migrants it is as simple as a search for food. The origins of long-distant migration patterns are more complex and include the development of the genetic make-up of the bird.
The mechanisms initiating migratory behavior vary and are not always completely understood. Migration can be triggered by a combination of changes in day length, lower temperatures, changes in food supplies, and genetic predisposition. Different species of birds and even segments of the population within the same species may follow different migratory patterns.
Migrating birds can cover thousands of miles in their annual travels, often traveling the same course year after year with little deviation in the path followed. First year birds may migrate unescorted to a winter home they have never before seen and return the following spring to the area in which they were born.
The secrets of their amazing navigational skills remain largely hidden. Birds appear to navigate using a variety of techniques, including navigation by the stars, sensing changes in the earth's magnetic field, and even smell.
Some species follow preferred pathways on their annual migrations. These pathways are often related to important stopover locations that provide food supplies critical to the birds' survival.
Each spring approximately 500,000 Sandhill Cranes and some endangered Whooping Cranes use the Central Platte River Valley in Nebraska as a staging habitat during their migration north to breeding and nesting grounds in Canada, Alaska, and the Siberian Arctic.
Taking a journey that can stretch to a round-trip distance of several thousand miles is a dangerous and arduous undertaking. It is an effort that tests both the birds' physical and mental capabilities. The physical stress of the trip, lack of adequate food supplies along the way, bad weather, and increased exposure to predators all add to the hazards of the journey.
In recent years long-distant migrants have been facing a growing threat from communication towers and tall buildings. Many species are attracted to the lights of tall buildings and millions are killed each year in collisions with the structures.
Scientists use several techniques in studying migration, from banding to satellite tracking. One of the goals is to locate important stopover and wintering locations. Once identified, steps can be taken to protect and save these key locations.
Migrating birds can sometimes be found in certain areas in larger than normal numbers. This may be the result of local weather conditions, an abundance of food or the local topography.
For example, small songbirds may migrate north in the spring, flying directly over the Gulf of Mexico to the coastlines of Texas and other Gulf Coast states. Under favorable weather conditions they may continue inland for many miles before stopping to rest and feed. However, storms and headwinds can often leave the birds near exhaustion when they reach land. In such cases they will head for the nearest location offering food and cover. Many of the motts along the gulf coast can provide these needs and become a temporary home to large numbers of birds in a very short time. These migration traps have become very popular with birders, even earning international reputations.
Giant live oak trees, like those found at High Island, Texas, attract many of our most beautiful birds as they reach the coast after their journey across the Gulf of Mexico each spring.
Spring migration is an especially good time for those that feed birds in their backyard to attract species they normally do not see. Offering a variety of food sources, water, and adding natural food sources to the landscape can make a backyard attractive to migrating songbirds.
Many birders use the range maps in their field guides to help determine if and when a particular species might be in their area. Range maps are especially useful when working with non-resident species.
Range maps can be confusing and have limitations. Ranges of birds can vary year-to-year, as with several of the irruptive species such as redpolls.
The range of some species can expand or contract fairly rapidly, with changes occurring in time periods shorter than the republication time of a field guide.
Introduced into the Bahamas in the mid-1970s, the Eurasian Collared-Dove is now established throughout the southeastern United States. Its range has rapidly spread north and west, all the way to the west coast of the United States. It prefers an urban or suburban habitat.
Migration and the spread of disease
Wild birds have historically played a very limited role in the spread of disease to humans. West Nile virus has had an impact on bird populations, with Corvid species such as crows and jays being the most susceptible. The mosquito is responsible for the transfer of West Nile virus to humans.
Bird or avian flu can also be carried by migrating birds and is often fatal to the bird. To date (May, 2006) there are no confirmed cases of the spread of avian flu from wild birds directly to humans.
Migration is a fascinating study and there is much yet to learn. The Lab of Ornithology's Miyoko Chu has published a book titled Songbird Journeys that explorers many aspects of migration in an interesting and easy-to-read style. The Cornell Lab of Ornithology's Handbook of Bird Biology provides even more information on the amazing phenomenon of bird migration.