Bird migration is defined as the regular movement of birds to and from a breeding to a non-breeding area. Reasons for migration can be ascribed to two main factors. These include primarily, food and a safe place to breed. Longer daylight hours and warmer temperatures provide an abundance of plants and insects the birds need to thrive and to provide for their young. As the temperatures drop and the days become shorter, the movement begins anew to return to southern wintering grounds to again take advantage of the plentiful food supplies.
In ancient times many beliefs were held about bird migration that we now find comical. For example, Aristotle believed certain birds transformed themselves into other birds to coincide with the seasons. A Roman naturalist, Pliny the Elder, believed that the Common Crane annually migrated to the far ends of the earth to do battle with Pygmies. Another common belief was hibernation. A sixteenth century Swedish archbishop, Olaus Magnus, presented the theory that swallows would congregate in huge numbers in the fall, sink down into the mud below the water and lie there packed like sardines until spring. Early explanations of migration may have been quite absurd, but human respect for this phenomenon of migration can be found in the Bible. Jeremiah stated that:
" Even the stork in the sky knows her seasons
And the turtledove, swift, and crane
Keep the time of their coming."
Another more current theory is that the ancestors of present-day migratory birds had their original homes south of their present nesting ranges. With the severe weather of fall and winter they retreated southward to their original homes, thus beginning the migratory habit. It is also speculated that the original homes of now migratory birds were in more northerly areas, which were warmer in past geological times. The changes in climate brought on by the advancing and retreating of glaciers, gradually forced some birds to migrate southward in order to survive. These as well as other theories reflect what might have happened, but it is almost certain that the migratory habit is older than the last glacial periods in North America and developed as a result of necessity.
It was not until the early nineteenth century that migration was accepted as the reason for the birds disappearance in the winter.
Migratory birds have evolved to take on their long distance flights efficiently. They begin to undergo physiological changes that will prepare them for the enormous energy cost on their bodies. In the period before migration, birds begin to eat more food in order to increase their fat reserves in order to provide energy for their long journey. Some birds will almost double their body weight by this fat storing process. The Ruby-Throated Hummingbird weighs only 4.8 grams but can use this stored fat to enable him to fly a non-stop, 24-hour flight across 600 miles of open water from the Gulf coast to the Yucatan Peninsula. An increase in eating also prepares the migrating birds’ feathers for the molting process. Different species molt at different times, but it is all for one purpose, and that is to insure that their feathers are in the best condition possible for their flights.
Migrations occur most often along established routes called “flyways”. There are four major North American flyways. They are the Atlantic, the Mississippi, the Central and the Pacific Flyways. These flyways tend to follow mountain ranges, rivers, or coastlines which may allow birds to take advantage of updrafts, thermals and other wind patterns. When a bird is flying into a headwind it will fly very low. Conversely, when a bird is flying with a tailwind, it will fly very high where the wind is the fastest. Soaring birds, such as birds of prey, will migrate during the day in order to make use of these thermals and updrafts, but since thermals do not form over water, these birds have difficulty crossing large bodies of water. Other bird species such as swallows and swifts will migrate during the day because of their ability to feed on insects as they fly. Most songbirds will migrate at night. They use the daytime to rest and search for food. These places where they stop are called stopover sites or staging areas. They will remain at these sites for different amounts of time based upon weather and how much their fat stores have been depleted and need replenishing.
Birds navigate by using a number of tools. They have excellent vision and rely on visual landmarks such as mountains, rivers, and coastlines to guide them. Birds also use the sun, stars and the earth’s magnetic field as compasses. They use the positions of the sun during the day and can use the setting sun as the indication of due west. Birds that fly at night use celestial navigation by knowing the patterns of the stars and by knowing special stars such as the North Star. In their first year of life, young birds memorize the position of the constellations in relation to the North Star. Since these patterns stay the same even though the Earth moves through space, it appears that the constellations move to different parts of the sky during the year. Birds have tiny grains of magnetite just above their nostrils. This mineral may help them to navigate using the Earth’s magnetic field, by telling the bird which direction is true north.
Human activities have impacted many migratory birds. Structures such as power lines, wind farms and off-shore oil rigs have affected migrating birds. Habitat destruction of the birds’ staging areas by draining wetlands and cutting down forests are particularly devastating to these tiny travelers.
The Migratory Bird Act of 1918 protected our migratory bird species from another kind of threat. It reads that the original 1918 statute implemented the 1916 Convention between the U.S. and Great Britain (for Canada) for the protection of migratory birds. Later amendments implemented treaties between the U.S. and Mexico, the U.S. and Japan, and the U.S. and the Soviet Union (now Russia). Specific provisions in the statute include: Establishment of a Federal prohibition, unless permitted by regulations, to "pursue, hunt, take, capture, kill, attempt to take, capture or kill, possess, offer for sale, sell, offer to purchase, purchase, deliver for shipment, ship, cause to be shipped, deliver for transportation, transport, cause to be transported, carry, or cause to be carried by any means whatever, receive for shipment, transportation or carriage, or export, at any time, or in any manner, any migratory bird, included in the terms of this Convention . . . for the protection of migratory birds . . . or any part, nest, or egg of any such bird." (16 U.S.C. 703).
This prohibition applies to birds included in the respective international conventions between the U.S. and Great Britain, the U.S. and Mexico, the U.S. and Japan, and the U.S. and the Russia.