Читать книгу Migration of Birds (1979) - Frederick Charles Lincoln - Страница 7

Table of Contents

The oldest, simplest, and most frequently used method of studying migration is by direct observation. Size, color, song, and flight of different species all aid the amateur as well as the professional in determining when birds are migrating. Studies begun by Wells W. Cooke and his collaborators (Cooke 1888-1915) and continued by his successors in the U.S. Bureau of Biological Survey (later U.S. Fish and Wildlife Service) were of particular importance in the earlier years of these investigations in North America. Some of the largest and most interesting routes and patterns were sorted out by tediously compiling and comparing literally thousands of observations on whether a species was or was not seen in a given locality at a particular time of the year. More recently, "The Changing Seasons" reports by many amateur bird observers in Audubon Field Notes (now American Birds) have been a most important source of information on direct observation of migration. In the aggregate, direct observation has contributed much to our knowledge of migration, but, as will be pointed out in other sections, until a few years ago, observers were not aware of some of the biases in this technique.

The "moon watch" is a modification of the direct observation method. It has long been known that many species of birds migrate at night. Until recently, it was not apparent just how important nocturnal migration really is. Significant information has been derived from watching, through telescopes, the passage of migrating birds across the face of a full moon. Since the actual percent of the sky observed by looking through a telescope at the moon is extremely small (approximately one-hundred thousandth of the observable sky), the volume of birds recorded is small. On a night of heavy migration, about 30 birds per hour can be seen. The fact that any birds are observed at all is testimony to the tremendous numbers passing overhead. Large-scale, cooperative moon-watching studies have been organized and interpreted by George H. Lowery, Jr. (1951; Lowery and Newman 1966).

Another specialized direct observation approach which has yielded important information on the spatial and altitudinal distribution of night migrating birds has been the use of small aircraft equipped with auxiliary landing lights (Bellrose 1971). Major disadvantages of night observation are that species cannot be identified and that birds continue to migrate without a full moon. However, these techniques do give information on the nocturnal migration movements that could not be obtained by other methods.