Each migratory species has its own characteristic route between its
nesting and winter ranges. These paths are often rather broad.
Waterfowl tend to follow a more restricted path, which can vary based on
availability of stopover habitat. It was thought at one time that birds
followed specific flyways, such as the Mississippi Flyway or Atlantic
Flyway. Banding studies have shown, however, that migrating songbirds
fly across broad areas and are not tightly grouped into specific
Nevertheless, some general patterns can be observed. In North America, many songbird and shorebird species follow an elliptical migration path. For example, a number of shorebird species that winter in South America will take a northern migratory path through Central America, the center of the United States and eventually to their summer homes in Northern Canada. In fall the birds fly southeastward, first to the Canadian Maritimes, followed by a long non-stop flight across the Western Atlantic Ocean to their wintering grounds. The route takes advantage of seasonal wind patterns that take some of the sting out of the long ocean stretch.
The third dimension
Birds do more than fly back and forth. Their migrations include a third dimension... how high they fly.
Different bird groups tend to fly at different altitudes during migration. Soaring migrants such as hawks and vultures usually look for the advantage that thermals offer and typically migrate at 3,000 feet or less. Migrating waterfowl use a wide range of altitudes, from as low as 300 feet to as high as 10,500 feet.
Most passerine species migrate at night. Over land, they usually fly at 2,100 to 2,400 feet but sometimes much lower. Over water, migration takes place at a much higher altitude, from 6,000 to 12,000 feet. Weather conditions often affect the migratory altitude as birds may fly higher or lower to avoid or take advantage of prevailing winds.
Some birds have been recorded at extremely high altitudes. Bar-headed Geese migrate over the Himalayas and have been recorded as high as 27,880 feet. It has been reported that a Ruepell's Griffin (a large African vulture) collided with an aircraft at 37,000 feet, or about 5 miles high!
The table below provides additional information on the flight height for several different species groups.
Individual birds show amazing consistency to their
migratory pathways and their nesting locations from year to year.
Banding studies indicate that birds may track their migration path
almost exactly from one year to the next and often return to the very
same field or nesting location. This site fidelity extends to stopover
points along the migration route and to wintering locations. Individual
Wood Thrushes, for example, winter in the same area each year in
Veracruz, Mexico, and demonstrate fairly consistent site fidelity in their U.S. breeding grounds.
Fidelity to stopover points is particularly visible among some of the larger bird species. Sandhill family groups are a good example. They gather into large flocks at traditional stopover points each year. A spectacular event occurs each spring when 80-90 percent of the mid-continent population of Sandhill Cranes stops in the North Platte and Platte River Valleys of Nebraska.
Important stopover locations
Shorebirds comprise another group that utilize key stopover
locations during migration. In 1974 the Manomet Center for Conservation
organized the International Shorebird Study. After studying over 600
sites, and with thousands of census reports, it became clear that many
species of shorebirds rely on a few, very important staging areas. The
loss of these areas or the related food supplies could be extremely
detrimental to shorebird populations.
A prime example is the current situation at Delaware Bay in New Jersey.
In mid-May, hundreds of thousands of shorebirds including Red Knots, Ruddy Turnstones, and Sanderlings stop on the shores of Delaware Bay to refuel before continuing their journey to their breeding grounds on the Arctic Tundra. The migrating birds stuff themselves on the eggs of horseshoe crabs before their final rush north.
In recent years the dynamics of the area have begun to change. Eel and whelk have become more popular food items in both the U.S. and other countries. Eels and whelk are also very fond of the eggs of the horseshoe crab. Fishermen have started taking large numbers of female horseshoe crabs to use as bait. Pick-up truckloads of horseshoe crabs can be seen leaving the beach. Some fishing boats have been equipped to drag nets across the sea floor, taking tens of thousands of horseshoe crabs in a single day.
The harvesting of the horseshoe crabs has dropped their populations significantly. In turn, a critical food supply for migrating shorebirds has been severely limited with the result that many shorebirds are not able to add the body fat needed to successfully complete their migration. About 80% of the North American population of Red Knots passes through Delaware Bay each spring. The lack of crab eggs has resulted in a precipitous drop in Red Knot populations. The result has been so severe that a coalition of conservation groups has petitioned the U.S. Fish and Wildlife Service to list the Red Knott as endangered. Some research indicates that the rufa subspecies of the Red Knot could be extinct by the year 2010 if steps are not taken to reduce the harvest of horseshoe crabs. Horseshoe crabs are relatively long-lived and do not start to reproduce until about ten years of age. Even if conservation efforts to rebuild crab populations are started immediately there will be a significant lag before the populations increase substantially.