The above two theories presume that the Quaternary glaciations, which occurred 10,000 to 1 million years ago, have been the predominate influence on bird migration in North America and Europe as we observe it today. There is no doubt these extreme climatic and ecologic barriers played a part in shaping or modifying some patterns, but as Moreau (1951) has pointed out, well-developed migrations occur in parts of the world, including the Southern Hemisphere or even within the tropics, where continental glaciation was not a factor. Furthermore, migrations to fit various needs have probably been going on ever since birds could fly. The tremendous Pleistocene glaciations actually occupied less than a hundredth of the time birds have existed on the earth and probably only determined the details of migrations as we see them today (Moreau 1951). The northern and southern ancestral home theories appear diametrically opposed to each other but Dorst (1963) concludes they are perfectly compatible. Since the phenomena probably occurred simultaneously, northern migrants then originated from two stocks: the North Temperate Zone birds sought refuge to the south during the glacial periods and the tropical avifauna expanded their range during the interglacial periods. Dorst also stated this double origin is more prevalent in North America where the tropical element is most abundant. Birds representing this element include hummingbirds, tyrant flycatchers, orioles, tanagers, and blackbirds. At some latitudes, they nest in the same area as the shorebirds which are of arctic parental stock. These theories assume migration is a genetic, inherited character, but we now know in some species it can be modified in the lifetime of one individual and the place some individuals return to nest or winter is not the ancestral home but a place to which they had been transported at an early stage in their development. Traditions that have lived for countless generations may die overnight if experienced individuals are lost or no longer active (Hochbaum 1955); migration patterns remain constant only as long as the factors influencing these patterns remain constant. But the landscape and the interacting ecological stresses are forever changing, and we would expect the adaptive behavior of birds to respond with them. One of these responses to an expanding habitat is colonization of new territory and expansion of a species' range with accompanying development of a migratory habit. The search for favorable conditions under which to breed in summer and to feed in winter, as influenced by competition for space, has been the principal factor underlying the extension of ranges, usually by young, nonconditioned individuals. This is exemplified by the northward extension in historic times of a number of species. Many of these range extensions have closely followed man's settlement of the area and the subsequent changes in habitat that man has made.