The ability to fly makes birds one of the most mobile species on earth. Man has admired and attempted to duplicate this flight characteristic for centuries, yet only with the development of jet and rocket engines have humans been able to surpass bird flight achievements. With flight comes the capability of long distance travel in a short period of time and the ability to select summer and winter habitats thousands of miles apart.
Avoiding cold temperatures and arctic winters, optimizing chances of survival, and enhancing productivity are strong motivators for migration. Even with this understanding I get excited when I read of a bird living in the perpetual long days of a summer climate -- north of the Arctic Circle during the Arctic summer and south of the Antarctic Circle during the Antarctic summer, or when I imagine a hummingbird flying non-stop across the Gulf of Mexico. Many marvels of bird migration exist. This lesson concentrates on migratory behavior and its physiological and navigational aspects.
Although migratory behavior is inherited, birds do not migrate without the proper hormonal stimuli. The pituitary and adrenal glands play an important role in stimulating a migratory bird to prepare for migration and to initiate migration. Just before fall migration, birds accumulate a thick layer of fat just under the skin, a response triggered by hormones. Birds can gain from 3-4 percent of their body weight each day. For a 200 pound human, this would be like gaining 6-8 pounds per day. For example, a Blackpoll Warbler weighing 11-12 grams will double its weight to 20-23 grams just before migration. This fat provides enough energy for the Blackpoll Warbler to fly nonstop for 85 hours over the Atlantic Ocean to its South American winter home. Once the fat layers are in place, cold weather will often initiate the actual departure from the summer range.
The more severe the climate the higher the percentage of species migrating. More Canadian species migrate than Mexico's nesting species. Food preferences and availability are definitely key contributors. Virtually all insect- and nectar-feeding birds migrate from the colder latitudes where insects and flowers are absent or extremely rare during winter months.
In the spring, hormones from the pituitary gland alter the physiology of a bird in preparation for the breeding season. Not only is the physiology altered, but a combination of hormonal changes also alter behavior patterns. Hormonal stimuli, longer day length and other links to the migration cycle are enough to initiate an exodus from the winter range. Migrating birds know to wait until their specific breeding habitat is reached before initiating the nesting process but pair bonding and other courtship behaviors will often be initiated along the route there.
Consider this. Each fall American White Pelicans leave Gunnison Island, within the Great Salt Lake, for a long trip south. Each spring they return to the island without the aid of a compass, map, or GPS satellites. Look how small Gunnison Island appears on a Utah map and imagine how small it would look on a North American map if it were even included among the features. How do pelicans find this small spot in the middle of a huge continent?
At first glance, results from many decades of research on bird navigation appears conflicting. Some studies have concluded that birds navigate by visual landmarks. Other studies conclude that birds rely on the position of the stars, moon, and sun. Still other scientists talk of the ability of birds to detect ultraviolet light patterns, the earth's magnetic field, and to use inherited cues for navigation. All are probably correct. Species adapt differently but usually use several navigational methods during their long trips. As we might use a compass, a map, signs, and previous knowledge to navigate to a desired location, birds also rely on more than one method.
A recent study suggests a link between the vision of birds and the ability to “see” the magnetic field to determine direction. Vision depends on photo receptors and one of the receptors, the one that makes you sleepy at night, also plays a role in magnetic field detection. I believe birds use this sense of direction along with other navigational cues to return to the same nesting area each spring. Many species will return to within a few feet of where they hatched or where they nested the previous season.
It's important to mention the different behaviors between day migrants and night migrants. Many birds that feed while traveling migrate during the day when feeding and traveling can be mixed. Day migrants include most hawks, eagles, cranes, pelicans, swallows, and vultures. Many small passerine birds rest by day and migrate by night. There has been much speculation on the advantages of night migration. To me, the most logical reasons are more safety from winged enemies, less turbulent night air, stars and moon to aid navigation and cooler temperatures.
Long distance flight is a very high energy consuming activity. As mentioned before, birds need ample food to be in good shape for migration. Larger birds (geese, pelicans, cranes, etc.) gain an energy advantage by flying in the familiar V formation, utilizing the up-draft turbulence from the leading birds. Smaller birds usually fly independently or in unorganized flocks. Most species vocalize while migrating. This vocalization not only helps keep the flocks and family groups together but gives us birders something to listen to during quiet spring and fall periods.
Migration distances vary from a few miles to a round trip of some 22,000 miles each year for the Arctic Tern. Barn Swallows and Common Nighthawks will travel some 4,000 miles from Utah to Brazil each winter. At the same time, Barn Swallows and Common Nighthawks nesting in the Yukon will travel some 7,000 miles to their Brazilian winter home. Swainson's Hawks regularly winter in Argentina, requiring a round trip of 11,000-17,000 miles each year. With the numbers of migrating birds and the distances they are traveling it is easy to grasp the importance of high quality habitat and abundant food along these migratory routes and at both destinations.
Migratory patterns are not fixed. Climatic, environmental, ecological, and human influences (feeding etc.) work together to cause some birds to lengthen or shorten their migration, to alter direction, to change routes, and in some cases to slowly eliminate migration all together as with the urban Canada Goose populations.