CIRCADIAN AND CIRCANNUAL PROGRAMS IN AVIAN MIGRATION

In migratory birds, endogenous daily (circadian) and annual (circannua l) rhythms serve as biological clocks that provide the major basis for their temporal orientation. Circannual rhythms are responsible for th e initiation of migration both in autumn and spring. This function of timing migrations is particularly important for birds that spend the w inter close to the equator where the environment is too constant or ir regular to provide accurate timing cues. In addition, circannual rhyth ms produce programmes that determine both the temporal and the spatial course of migration. In Silvia warblers, the time programmes controll ing autumn migration are organized in a species- or population-specifi c manner. It has been proposed that, in first-year migrants, the time programme for autumn migration plays a major role in determining migra tory distance, thus providing the vector component in a mechanism of v ector navigation. It is not yet clear, however, whether this programme does indeed determine migratory distance or whether it only provides the temporal framework within which other factors determine how far a bird flies. Evidence against the first alternative comes from findings indicating that migratory activity can be drastically modified by a c onstellation of rather specific, but highly relevant, factors and that the resulting changes in migratory activity are not compensated by su bsequent increases or decreases of migratory activity. In normally day -active but nocturnally migrating birds, circannual signals cause alte rations in the circadian system leading to the development of nocturna l activity. Although the nature of these signals is unknown, there is evidence that changes in the diurnal pattern of melatonin secretion by the pineal gland are associated with, and possibly causally involved in, the waxing and waning of nocturnal activity. These changes in the melatonin pattern presumably also affect general synchronization prope rties of the circadian system to Zeitgebers in such a way that circadi an rhythms adjust faster to new conditions after long transmeridian fl ights.