The Migration Of Birds

Ever wonder how birds migrate? The migration of birds is part of an animal's life that is characterized by geographic movements. Find out the different types of migration and if birds have an internal compass in this article.

Migration occurs wherever there are birds. Migration is the part of the animal's life that is characterized by geographic movements. This is an incredibly diverse behavior, an adaptation that has been shaped by natural selection. Arctic birds migrate, as do tropical birds. We often think of migration as the seasonal movement of birds during spring and fall to avoid harsh winters. We have all heard the phrase "fly south for the winter." This is only partly correct. Scientists have extensively studied the evolution of migration. Not one theory has been widely accepted. Explanations include and are not limited to glaciers, changing climatic patterns, and extending dispersal distance. Most biologists

will agree that the foundations of all explanations must be food. The availability of food is the driving force in the evolution of migration. Food abundance can increase

reproductive output, whereas a lack of food leads to death A bird that can find food will

have a longer life span and produce offspring more readily than one that finds less food.

Just by moving from place to place a bird can find more food, and thus migration evolved. It just so happens that many warmer climates have an overabundance of food, whereas cold harsh environments do not. Most species of birds can tolerate colder temperatures given that food is available.

Scientists have identified three patterns of migration: complete, partial and

irruptive. Complete migration occurs when individuals leave the breeding range during

the nonbreeding season. Many North American birds such as warblers, orioles, hummingbirds, and shorebirds exhibit this migration pattern. Complete migrants may travel incredible distances, sometimes more than 15,000 miles per year. The most common type of migration is partial migration. This is characterized by seasonal

movements away from a breeding range by some but not all of the members of a species.

For example, Song Sparrows migrate south for the winter, but some individuals remain in

the breeding area. Thus the migration is not complete across the species, instead it is

only partial. Partial migrants, like complete migrants, take advantage of seasonally

abundant food. Migrations that are not seasonally or geographically predictable are said to be irruptive. Such migrations may occur one year but not again for many years. The distances and numbers of individuals involved are also less predictable than with complete or partial migrants. The Great Gray Owl is an irruptive migrant. It migrates south only occasionally and the number of these owls that migrate vary. Many scientists call these irruptive migrants food specialists. Many of these migrants may only eat

certain seeds from certain trees. When the seeds are available, irruptive migrants stay;

when the seeds are difficult to find, they move.

The factors that are believed to control the onset of migration can be divided roughly into external (exogenous) or internal (endogenous) factors. The seasonal timing of migration can be controlled by both at different times. Migration onset and seasonal timing are controlled genetically in some species. This would be an example of an endogenous factor. In other species, the onset of migration is controlled by weather, food, or social factors. These would be examples of exogenous factors. It is difficult to study endogenous factors such as genetic control.

How do birds find there way to their destination? Studies have shown that migratory birds find their way over long distances, which implies that these birds must have a sort of mental compass and map. The migratory pathways used by birds are influenced by topography, geography and wind. These routes are not always the same from year to year. Bodies of water, deserts, and mountain ranges are the physiographic features of the earth that can act as barriers. Many researchers have concluded that wind is one of the strongest factors in determining a migration pattern. Many birds rely on the direction of the wind to get them there. This is also true in airplanes. If the wind is in your favor, your flight will be faster. Research has revealed that many migratory birds take advantage of winds to make better progress. Many scientists believe that a bird has an internal compass that permits orientation in a given direction. Many researchers have thought that birds rely on olfactory or magnetic cues to guide them to their destinations. Another hypothesis suggests that birds have an innate time program coupled to an innate flight direction. Birds are thought to be genetically programmed to fly in a given direction for a given period of time. Some studies have proven this theory to be correct; only more research and time will tell.

How do scientists study migration? One of the most popular methods of studying migration is banding birds. Banding is a four-part process. First scientists catch the birds. Then they band the birds (usually a leg band encoded with a number and other information). They then release the birds into the wild. Finally they transfer the information into a database. Banding does have its shortcomings. For starters, most birds that are banded are never seen again. Birds are hard to track. Another method that has been used is radar. Most radars are used to examine the movement of large numbers of birds overall several hundred square kilometers. The position of each bird can be recorded every few seconds and its speed, altitude, direction and mode of light are measured with precision. Radar is a wonderful tool and the only shortcoming appears to be that it is effective for only a small area. Radiotelemetry is another technique. This involves putting radiotransmitters on birds so that their movements and activities can be monitored from a distance. The transmitters weigh less than three percent of a bird's body weight, and birds tend to adjust to them rapidly. The transmitter can send out a signal every few seconds. Most transmitters are battery powered, but some are solar powered. Once a transmitter is attached to a bird, the researcher follows it from an automobile or airplane fitted with an antenna to receive the transmitted signal. Data gathered through radiotelemetry is more detailed than other methods of study. The physiological and neural mechanisms that control a migrant's behavior can be studied in migration laboratories that are designed to duplicate the migratory environment. A researcher will often create a controlled environment that stimulates one or more aspects of migration, than manipulates them experimentally. Orientation, navigation, and biological rhythms are the most often studied in laboratories. All such methods are extremely helpful in uncovering the mystery of migration. Even though much research has been done in this field, much more still needs to be completed. There are many unknowns when it comes to migration. Most of the information we have is based on theories that have been demonstrated experimentally. This makes it hard to imagine any such theory on migration being set in stone. Migratory behaviors and routes can change and this is why it is often frustrating for scientists. The next time you see a bird flying overhead, you may ponder where they are going and how many miles they will travel in order to get there.

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