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In three species of birds that migrate long distances, the annual rhythms of gonadal activity, molt, and migratory restlessness ( Zugunruhe ) persist for more than 1 year under certain constant conditions. The most important zeitgeber for these circannual rhythms is the annual cycle of photoperiod, which adjusts the overall period of circannual rhythms to exactly 1 year and also provides for the appropriate adjustment of seasonal activities to the temporal structure of the environment. This is illustrated by results on garden warblers ( Sylvia borin) indicating that the longer photoperiods experienced by individuals wintering far south in the African wintering area phase-advance spring migration and the accompanying gonadal devel opment, relative to those of individuals wintering further north. The rate of acceleration is, however, slow enough to prevent a reproductive cycle during the Southern Hemisphere summer. Hence, endogenous circannual components and zeitgeber stimuli constitute a func tional entity that provides as a whole for adaptive temporal programming. This idea is further supported by findings in the pied flycatcher ( Ficedula hypoleuca ), in which a circannual rhythmicity persists only if photoperiod in winter is at least as short as that normally encoun tered by the species in its wintering grounds slightly north of the equator. In collared fly- catchers ( Ficedula albicollis), in contrast, rhythmicity continues under much longer photo periods, consistent with the fact that the wintering area of this species extends to latitudes far south of the equator. It is proposed that the adaptive function of circannual rhythms can be properly understood only if their interactions with environmental factors, particularly those that play a role as zeitgebers, are analyzed in sufficient detail. The biological significance of circannual rhythms may be more apparent in the context of the environmental constraints limiting their expression than in the often rather restricted set of conditions sustaining spon taneous annual cyclicity.

Photoperiod as a Modifying and Limiting Factor in the Expression of Avian Circannual Rhythms