Much has been learned about the proximate control of migration from studies on captive birds. Under natural daylengths, caged birds from obligate migratory populations develop fat reserves and migratory restlessness at appropriate dates in autumn and spring, at about the same times as their wild counterparts. Evidently, the same factors that stimulate departure in wild birds trigger restlessness in captive ones kept on natural daylengths (Gwinner 1972, Berthold 1996).
The role of daylength in promoting these processes (in association with an endogenous rhythm) is shown by findings discussed in detail later, namely that: (1) captive birds experimentally exposed to photoperiods longer than natural days in spring develop migratory condition earlier than their wild counterparts; and (2) birds exposed to photoperiods shorter than natural days in late summer or autumn also develop migratory condition earlier than their wild counterparts.
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