Migration is too complex to be explained except historically. (A. Landsborough Thomson 1936.)
It is comparatively easy to trace the probable steps in the evolution of the migrations of some species, and some routes have developed so recently that they still plainly show their origin. (Frederick C. Lincoln 1935.)
Some bird species have changed their migratory habits in recent decades, but others have not, although it seems to the human onlooker that they would benefit from doing so. Yet other species undertake such long journeys over sea or desert that one wonders how their migrations could have arisen. This chapter examines some of the more puzzling migration patterns of birds in order to understand how they might have evolved, and why they continue to persist. Some extraordinary journeys can be explained most plausibly in terms of past conditions which may have influenced their development.
Five aspects are considered here: (1) indirect migration routes and seemingly unnecessarily distant wintering areas; (2) long migrations over seas or other hostile areas; (3) loop migrations; (4) migratory divides; and (5) the development of migration, from lower to higher latitudes, as well as from higher to lower latitudes.
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