CIRCANNUAL CLOCKS IN AVIAN REPRODUCTION AND MIGRATION

Many behavioural and physiological functions of organisms are adjusted to the periodic changes in their environment, particularly to those r elated to the natural day and year. This adjustment is often achieved through the action of endogenous daily (circadian) and annual (circann ual) clocks. Studies of the control of avian moult, migration and repr oduction have played a major role in understanding how biological cloc ks function and interact with rhythms in the environment. Investigatio ns on tropical birds such as the East African subspecies of the Stonec hat (Saxicola torquata axillaris) and long-distance migrants like the Garden Warbler (Sylvia borin) have provided the longest records of cir cannual rhythms, some of them running for more than 12 years, with per iods ranging from about 9 to 13 months. Avian circannual rhythms are o rganized in a characteristic way for a particular species or populatio n, and cross-breeding experiments have shown that some of the differen ces found among them are genetically determined. In African Stonechats circannual rhythms guarantee that seasonal events occur at the approp riate times of the year and in the characteristic sequence. They also control a ''reproductive window'' that provides the temporal framework for breeding. The width of this window is rather inflexible but the p erformance of a bird within this framework (e.g. whether it breeds onc e or twice per season) is subject to modification by environmental con ditions. In migratory birds circannual programs are involved in determ ining the time course, distance and direction of migration. Circannual rhythms are synchronized with and modified by environmental factors i n a complex way, but the endogenous mechanisms usually respond to envi ronmental cues such that an optimal adjustment to season and latitude is guaranteed.