Aristotle also was the originator of the theory of transmutation, or the seasonal change of one species into another. Frequently one species would arrive from the north just as another species departed for more southerly latitudes. From this he reasoned the two different species were actually one and assumed different plumages to correspond to the summer and winter seasons.
Probably the most remarkable theory advanced to account for migration is contained in a pamphlet, "An Essay toward the Probable Solution of this Question: Whence come the Stork and the Turtledove, the Crane, and the Swallow, when they Know and Observe the Appointed Time of their Coming," mentioned by Clarke (1912: v. 1, 9-11) published in 1703. It is written "By a Person of Learning and Piety," whose "probable solution" stated migratory birds flew to the moon and there spent the winter. Astronauts have so far failed to verify this.
Some people, who easily accepted the migratory travels of larger birds, were unable to understand how smaller species, some of them notoriously poor fliers, could make similar journeys. They accordingly conceived the idea that larger species (e.g., storks and cranes) carried their smaller companions as living freight. In some southern European countries, it is still believed these broad-pinioned birds serve as aerial transports for hosts of small birds that congregate upon the Mediterranean shore awaiting the opportunity for passage to winter homes in Africa. Similar beliefs, such as hummingbirds riding on the backs of geese, have been found among some tribes of North American Indians.
Today we realize that birds do not migrate by "hitching" rides with other birds and that the scope of the migration phenomenon is worldwide, not simply limited to the United States, the Northern Hemisphere, or the world's land masses. The migration heritage is developed just as extensively in Old World warblers migrating to and from Europe and Africa as in our wood warblers traveling from Canada and the United States to South America and back. One of the fundamental differences in migration patterns of the Northern and Southern Hemispheres is that no land species nesting in the South Temperate Zone migrates into the North Temperate Zone, but a few seabirds, such as the sooty shearwater, Wilson's storm-petrel, and others, migrate north across the Equator over the vast ocean expanses after nesting in the South.
TECHNIQUES FOR STUDYING MIGRATION
Before we discuss the many intricacies of how, when, and where birds migrate, one should have a general idea of how migration data are collected and what methods are currently being used to increase our knowledge. Since this publication first appeared in 1935, many new procedures have been used in the study of bird migration. One of these, radar, has been an invaluable adaptation of a technique developed for a quite different, but related, purpose.
Direct Observation
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.