Читать книгу Migration of Birds (1979) - Frederick Charles Lincoln - Страница 10

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If birds can be captured, marked, and released unharmed, a great deal of information can be learned about their movements. Many different marking methods have been developed to identify particular individuals when they are observed or recaptured at a later date. A few of the general methods are summarized in this section.

Bands, Collars, Streamers

Since 1920, the marking of birds with numbered leg bands in North America has been under the direction of the U.S. Fish and Wildlife Service in cooperation with the Canadian Wildlife Service. Every year professional biologists and voluntary cooperators, working under permit, place bands on thousands of birds, game and nongame, large and small, migratory and nonmigratory, with each band carrying a serial number and the legend, NOTIFY FISH AND WILDLIFE SERVICE, WASHINGTON, D.C., or on the smaller sizes, an abbreviation. When a banded bird is reported from a second locality, a definite fact relative to its movements becomes known, and a study of many such cases develops more and more complete knowledge of the details of migration.

The records of banded birds are also yielding other pertinent information relative to their migrations such as arrival and departure dates, the length of time different birds pause on their migratory journeys to feed and rest, the relation between weather conditions and starting times for migration, the rates of travel for individual birds, the degree of regularity with which individual birds return to the summer or winter quarters used in former years, and many other details. Many banding stations are operated systematically throughout the year and supply much information concerning the movements of migratory birds that heretofore could only be surmised. The most informative banding studies are those where particular populations of birds are purposely banded to produce certain types of information when they are recovered. Examples of such planned banding are the extensive marking of specific populations of ducks and geese on their breeding grounds by the U.S. Fish and Wildlife Service and the Canadian Wildlife Service, as well as in "Operation Recovery," the cooperative program of banding small landbirds along the Atlantic Coast (Baird et al. 1958). When these banded birds are recovered, information concerning movements of specific populations or the vulnerability to hunting is gained. Colored leg bands, neck collars, or streamers can be used to identify populations or specific individuals, and birds marked with easily observed tags can be studied without having to kill or recapture individuals, thus making it a particularly useful technique.

We have learned about the migratory habits of some species through banding, but the method does have shortcomings. If one wishes to study the migration of a particular species through banding, the band must be encountered again at some later date. If the species is hunted, such as ducks or geese, the number of returns per 100 birds banded is considerably greater than if one must rely on a bird being retrapped, found dead, etc. For example, in mallards banded throughout North America the average number of bands returned the first year is about 12 percent. In most species that are not hunted, less than 1 percent of the bands are ever seen again.

In 1935, Lincoln commented that, with enough banding, some of the winter ranges and migration routes of more poorly understood species would become better known. A case in point is the chimney swift, a common bird in the eastern United States. This is a nonhunted species that winters in South America. Over 500,000 chimney swifts have been banded, but only 21 have been recovered outside the United States (13 from Peru, 1 from Haiti, and the rest from Mexico). The conclusion is simply this: Whereas banding is very useful for securing certain information, the volume of birds that need to be banded to obtain a meaningful number of recoveries for determining migratory pathways or unknown breeding or wintering areas may be prohibitive. One problem in interpretation of all banding results is the fact that recoveries often reflect the distribution of people rather than migration pathways of the birds.

Other methods used to mark individuals in migration studies include clipping the tip end off a feather (not a major flight feather) with a fingernail clipper or touching the feather with colored paint or dye. This marking technique is obviously good for only as long as the bird retains the feather (usually less than one year), but allows the investigator to recognize whether the bird has been handled previously or not.