There is also infinite variety in the routes covered during migration by different species. In fact, the choice of migration highways is so wide that is seems as if the routes of no two species coincide. Differences in distance traveled, time of starting, speed of flight, geographical position, latitudes of breeding and wintering grounds, and other factors contribute to this great variation of migration routes. Nevertheless, there are certain factors that serve to guide individuals or groups of individuals along more or less definite lines, and it is possible to define such lines of migration for many species.
Except in a few species, individuals probably do not follow precisely the same route twice. This is especially true in the group of soaring birds that utilize thermals. Mueller and Berger (1967b) recaptured only three migrants in subsequent years at Cedar Grove, Wisconsin, after banding over 50,000 birds there. In general, those populations of species with very discernible breeding or wintering grounds have readily discernible migration routes. However, even the whole migration process of certain species may show great yearly fluctuations (Rudebeck 1950).
Aldrich et al. (1949) showed from banding data great variation in migration patterns between species of waterfowl. In some species there was considerable diversity in direction of movement, not only of different breeding populations within a species but also for different individuals of the same breeding population. The impression is inescapable; waterfowl migration is even more complicated than originally supposed, and it is difficult to make generalizations with regard to migration pathways for even a single species let alone waterfowl in general.
Through plotting accumulated banding data in the 1930's, investigators became impressed by what appeared to be four broad, relatively exclusive flyway belts in North America. This concept, based upon analyses of the several thousand records of migratory waterfowls recoveries then available, was described by Lincoln (1935a). In this paper (p. 10), Lincoln concluded that:
... because of the great attachment of migratory birds for their ancestral flyways, it would be possible practically to exterminate the ducks of the West without seriously interfering with the supply of birds of the same species in the Atlantic and Mississippi flyways, and that the birds of these species using the eastern flyways would be slow to overflow and repopulate the devastated areas of the West, even though environmental conditions might be so altered as to be entirely favorable.