Link to ecology.com Home Page
Earthworks & Systems, Human Impact & Home

On the Move!
Bird Migration Holds Clues to Public Health

Eric McLamb
September/October 2001
About 2-3 billion birds migrate from northern regions to the south each year as part of the circle of survival. Migrations provide clues about the health of our planet in ways yet to be understood. (Courtesy: US Coast Guard)

As winter approaches, billions of birds worldwide are flying mostly south from their northern homes for where food is plentiful and living conditions are hospitable. Like clockwork, these birds depart for their winter homes as a means of survival, despite the fact that their journeys can be quite formidable.

Migrating birds depend on the seasonal availability of resources in order to survive and maintain their health. In Russia, for example, the Russian Arctic offers swans an abundance of food in the summer which can be gathered with less competition from other species, and safe nesting places where there is relative freedom from human disturbance.
Gray whales migrate almost 14,000 miles round trip each year as they travel form the Bering Straits to the Sea of Cortes between Baja California and Mexico. (Courtesy: California Academy of Sciences)

After summer, as the food supply disappears and the ice begins to permeate the region, the birds migrate to milder climates such as Great Britain where they can find ample resources and shelter to sustain them.

Migration is one of the most widely studied areas of bird biology, and yet it is very poorly understood. Even though people have observed and noted migrations for centuries, there is sparse information and few theories about how birds accomplish such impressive flights each year. But, what is becoming clearer is that these migratory birds are harbingers of the health of our planet, providing clues to changes in the Earth's systems that affect the human condition and overall public health.

Many animals migrate, including whales, fish, butterflies, turtles and numerous species of antelope (i.e., wildebeest, caribou). And while many of these animals travel incredible distances, like the gray whale which travels 10-14,000 miles round trip, no animal travels as far and through as much adversity as many bird species.
The Arctic Tern has the longest migration route of all animals, from the North Pole to the South Pole and back each year. (Courtesy: US Geological)

In fact, the Arctic Tern accomplishes the extraordinary task of flying from the North Pole to the South Pole -- and back again -- each year in a route covering about 22,000 miles! And while migration patterns in North America are generally north and south, in Europe, a number of migrations occur more east-west.

So why do these species go through so much effort and peril. For these birds, it's all about survival, and their survival depends on the state and conditions of the natural world. Just as they have habitats on which they depend in one region, they equally depend on habitat in another place that range anywhere from 300 miles to 10,000 miles apart. They begin to migrate when their biological clock, which is determined by the length of the day, tells them it's time to go. But other environmental factors such as the weather, their state of nutritional health and their interaction with other birds enable them to pinpoint exactly when their migration should begin.
Bar-headed geese fly across the Himalayan Mountains at 29,000 feet, higher than Mount Everest. (Courtesy: British Water Fowl Association

Although birds can ride out extreme weather conditions and threats from natural predators, their greatest threat comes from loss of habitat, mainly due to human development and related activities. Forests and wetlands are vital to birds' survival because they provide food and water, shelter, protection from predators and places for rest and food during their migrations. Over the past 100 years, as human populations surged and industrial and technological progress was made, much of the forests and wetlands have been depleted and thus seriously changed the landscape and resources for these migrating birds. As a result, bird numbers have been seriously affected in many parts of the world.

Of the 9,600 known bird species, nearly 1,200 are threatened with extinction (source: Johns Hopkins Center for Communications Program). About 99 % of the globally threatened birds are at risk from human activities such as agriculture, logging, and other major changes in the world's ecosystems. Hunting and trapping are also contributors, but pale in comparison to changes brought upon the ecosystems. These ecosystems provide vital services (such as maintaining global climate patterns, mediating the carbon cycle, safeguarding watersheds and stabilizing soils), valued at $33 trillion per year (Source: BirdLife International, United Kingdom). The potential loss of large numbers of species facing extinction is a powerful indication that the quality of these ecosystem services is deteriorating.

In North America, for example, bird observers have seen a steady decline in the numbers of many of the birds which migrate to Central and South America. And though deforestation or problems with their summer breeding habitats were suspected to be the problems, they also realized that forests were not being lost as fast as the rate of the disappearance of the birds.

A recent study on regional forest fragmentation pinned much of the blame on urban sprawl and development which are significantly altering and removing valuable ecosystems on which birds depend. This is not only happening in the United States, but it is happening worldwide. In the US alone, this affects 80 percent the total bird population since about 520 of the US' 650 bird species migrate.

Importance of Wetlands
Snow geese will fly all day and night, despite all conditions, until they reach the next suitable wetland area where they feed off of the grass and small fish. Such areas have become more and more scarce in the past century, resulting in reduced numbers of snow geese. (Courtesy: National Audubon Society)

Wetlands are areas that link water and land. They include a wide range of areas from marshes and swamps to areas between dry land and rivers, streams, lakes and coastlines. Though they are not necessarily wet year-round, they harbor very rich nutrients for plants and animals, including insects which are a primary food for birds. Thus, wetlands provide vital habitat for many species of plants and animals, including about half of all known bird species.

But wetlands also provide needed protection of property and water quality vital for humans. They act as a sponge to absorb flood waters from nearby streams and rivers or ocean tides, for example, and they filter out impurities and pollutants that could flow into main water sources. Wetlands also serve to clean the air of carbon dioxide, which is absorbed by plants. Carbon dioxide is the substance which enables photosynthesis in plants, the process by which solar energy is converted into food and fiber necessary for plant growth and health. As wetlands become fragmented and disappear, the domino effect extends directly to both humans and wildlife and their respective qualities of life.

Tracking Bird Migrations

Birds live on more than 20 percent of the Earth's surface, but about three-fourths of the threatened or endangered birds inhabit less than five percent of the land. This enables most bird species to be easily tracked, particularly threatened species, and also allows scientists, conservationists and property owners to focus their attention where extinction risks are the highest. Generally, these tracking and planning activities can simultaneously focus on threats to the environment since they are usually at the heart of risks to bird populations.

Birdwatching is one of the world's most treasured pastimes. According to the US Fish & Wildlife Service, 76 million Americans are actively engaged in the sport of bird watching -- or birding, making it the second largest leisure time activity just behind gardening. It is estimated that Americans spend over $12 billion each year on bird seed and related equipment, making birding not only a major hobby but big business (Source: The Backyard Birder).
Radar can effectively track bird concentrations in limited regions. The image above shows geese migration in the mid-Atlantic area of the US. (Courtesy: Clemson University)

Yet, birding is very important for studying birds and migrations, particularly small migratory birds. There are literally tens of thousands of reporting groups, Internet sites and other forums used by birders to report their findings. Other methods of reporting bird statistics, particularly those of threatened or endangered species, include radar and bird banding (ringing). While radar may be useful in determining mass movements of birds, it is limited in its range and value of information detail collected. The Clemson University Radar Ornithology Laboratory (CUROL) is one of the worlds' foremost experts in tracking birds by radar.

Traditionally popular among scientists and conservationists, bird banding involves the attachment of a band, or ring, on a bird -- usually placed harmlessly and painlessly around the bird's leg -- which carries special identification (serial number) and tracking information unique to the bird. When they are located, the serial number assigned to the bird is sent to the US Fish & Wildlife Service Bird Branding Laboratory where the information is recorded.

Each year, over a half-million birds are banded by scientists in North America. This is significant because bird banding provides ornithologists with valuable information about the birds, their migration patterns health and other information which help determine what they need in order to survive. With this information, ornithologists can best work with conservationists and other scientists and policy makers in creating sound conservation policy.

But a new form of tracking birds began to emerge in the 1980s involving satellites and tiny transmitters attached to the birds themselves. Information about each bird's location and activity is beamed to a satellite which is then stored by the spacecraft's data collection system. This information is then sent to ground stations on Earth and ultimately is sent to NASA's Goddard Space Flight Center in Maryland for analysis. The information collected include such vital information as the location of the bird, body and ambient temperature, flight speed, etc. The NASA satellite tracking system provides the information to researchers at Patuxent Wildlife Research Center who are studying the migration routes and habitats of the birds. The data is also used to pinpoint the exact locations of birds and their level of activity.

Armed with the comprehensive information now being collected about the world's migratory birds, scientists hope to discover clues and develop solutions relating to Earth's environmental health. Yet, this information will be effective in helping individuals understand the true inter-connectivity of all life which will lead to sound policy for sustainable development.

Did You Know?

  • Most long distance migrant birds fly at night. They may travel continuously or land daily around sunrise to rest and feed.

  • Pigeons are known to navigate by electromagnetic fields present in the earth's crust while sparrows are known to navigate by certain stars.

  • A bird's heart beats 400 times per minute while resting and up to 1000 beats per minute while flying.

  • The world's only wingless bird is the kiwi of New Zealand.

  • The American turkey vulture helps human engineers detect cracked or broken underground fuel pipes. The leaking fuel smells like vulture food (they eat carrion), and the clustered birds show repair people where the lines need fixing.

  • North American blue grouse migrates over 300 miles, from deciduous woodland in summer to pine woodland for winter. It is one of the shortest bird migrations.

Other Sources of Information