Circannual Rhythms in Bird Migration: Control of Temporal Patterns and Interactions with Photoperiod

Abstract

The idea that endogenous timing mechanisms may play an important role in the control of avian migrations is about as old as the insight that birds do indeed migrate. As early as 1702 Baron von Pernau (1702) suggested that migratory birds were “driven at the proper time by a hidden drive” (“durch einen verborgenen Zug zur rechten Zeit getrieben”), and similar propositions were made by other early investigators of migrations like Naumann (1822); Brehm (1828), and von Homeyer (1881). The participation of endogenous factors in the timing of migrations seemed especially likely in long-distance migrants that spend the winter close to the equator. These birds molt and start homeward migration at rather well-defined times in winter and early spring, in spite of the apparent absence of regular seasonal environmental changes. In view of this situation even Rowan (1926), who was so successful in explaining many aspects of avian annual cycles on the basis of photoperiodic effects, could not escape the conclusion that “those species that breed in the northern hemisphere and winter on the equator or cross it and winter in the southern hemisphere, make necessary the assumption that there is another and internal factor, a physiological rhythm”.