Annual Periodicity in the Migration and Reproduction of Birds

The term "annual periodicity " is of course a mis-nomer. No animal periodicity could be precisely an-nual. Nevertheless, the kind of periodicity that I want to discuss is a relatively lengthy one, and in some cases it may be sufficiently precise to govern the time of movement to the breeding rounds and thus indirectly influence the time of reproduction. At the outset it is essential to separate the factors that initiate the pre-nuptial movement from those that culminate reproduction. Theformer factors merely ensure that migrants or nomads will start back toward their traditional breeding grounds in time to come under the control of the latter factors, i.e., the external stimuli that operate there and bring the sexual cycle to culmination. For example, daylength initiates the