------------------------------------------------- Physiology of Migration ------------------------------------------------- The physiological basis for bird migration has received considerable attention, particularly the effects of seasonal increases and decreases in daylight and the seasonal rhythms occurring within animals and referred to as "biological clocks." Investigations in this field include the pioneering work on the relationship of photoperiod (daylength) to migration by Rowan (1925, 1926) and many subsequent studies (Wolfson 1940, 1945; Marshall 1961; King, Barker and Farner 1963; King and Earner 1963; King 1963; Farner 1955, 1960; and Farner and Mewaldt 1953). These studies have become ever more deeply involved in the intricate relationships between photoperiod, endocrine interactions, gonad development, fat deposition, and migratory unrest. They add to our knowledge of the mechanisms that regulate the migratory behavior we observe. =ADVANTAGES OF MIGRATION= Why should a bird subject itself to the rigors of a long migratory journey twice a year if it can find all the requirements suitable for existence in one locality? It seems well to consider briefly the ends that are served by this annual round trip between breeding grounds and winter quarters. Obviously, the migratory habit enables a species to enjoy the summer of northern latitudes and to avoid the severity of winter. In other words, migration makes it possible for some species to inhabit two different areas during the seasons when each presents favorable conditions. If it was not advantageous to make the trip twice a year, natural selection would have eliminated the tendency, but bird migration has become the rule over much of the world rather than the exception. By withdrawing in the spring to regions uninhabitable earlier in the year, migrant species are generally assured of adequate space and ample food upon their arrival in the winter-freed North, and those nonmigratory kinds, which stay behind to nest, are also assured of ample space for these activities. Every pair of birds requires a certain amount of territory for the performance of its reproductive duties, the extent of which varies greatly between different species. This territory must be large enough to provide adequate food, not only for the parent birds but also for the lusty appetites of their young. In the Arctic summer, 24 hours of daylight allow the young to feed or be fed almost continuously and rapid growth is apparent. The short breeding season in northern latitudes exposes the vulnerable young to predation for a brief period and prevents a build up of predator populations.