Migrations of birds are conspicuously rhythmic phenomena. As a rule, they occur at certain seasons and certain times of day and thus are manifestations of an approximately annual (circannual) as well as an approximately diurnal (circadian) rhythmicity. Both of these rhythms reflect basic adaptations to environmental cycles and serve as biological clocks to cope with the annual and daily fluctuations in external conditions (see reviews in Gwinner, 1990b, 1996; Berthold, 1993). Taken together, the annual and daily clocks of migratory birds provide the major basis for their 'orientation in time' supplementing their 'orientation in space'. These two categories of orientation mechanisms, although controlled by separate processes, interact at various levels to guide birds on their migrations. The present account summarizes some recent results on the temporal orientation of migratory birds, concentrating on areas in which mechanisms of orientation in time interact with mechanisms of orientation in space. Emphasis will be placed on defining open questions and clarifying some conceptual issues, rather than exhaustively reviewing the field. The endogenous nature of annual and daily rhythms in migration Many bird species, such as warblers, migrate exclusively at night. When kept in cages provided with an appropriate arrangement of perches, the birds' locomotor activity can be measured by means of microswitches mounted underneath the perches. With this widely used experimental method, it has been shown that locomotor activity in summer and winter is restricted to daytime, i.e. it occurs exclusively in the light portion of the light:dark (L:D) cycle. In autumn and spring, however, the seasons corresponding to natural migratory activity, the birds exhibit additional locomotor activity at night. Fig. 1 illustrates for two garden warblers (Sylvia borin; A and B) that even when the light:dark cycle is experimentally kept constant over the year – and can thus provide no cues about the season – the birds exhibit nocturnal activity (termed 'Zugunruhe' or migratory restlessness) in autumn and spring that approximately coincides with the actual migratory seasons of free-living birds of the same species. These and other experiments have shown that rhythmic waxing and waning of nocturnal activity indeed persists for many cycles in constant conditions. The rhythm is usually accompanied by variations in migratory fattening (indicated by an increase in body mass) and followed by a moult in winter and a phase of reproductive activity in summer (see Fig. 1). In the typical case, the period of the rhythm is longer or shorter than 12 months under such In migratory birds, endogenous daily (circadian) and annual (circannual) rhythms serve as biological clocks that provide the major basis for their temporal orientation. Circannual rhythms are responsible for the initiation of migration both in autumn and spring. This function of timing migrations is particularly important for birds that spend the winter close to the equator where the environment is too constant or irregular to provide accurate timing cues. In addition, circannual rhythms produce programmes that determine both the temporal and the spatial course of migration. In Sylvia warblers, the time programmes controlling autumn migration are organized in a species-or population-specific manner. It has been proposed that, in first-year migrants, the time programme for autumn migration plays a major role in determining migratory distance, thus providing the vector component in a mechanism of vector 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 constellation of rather specific, but highly relevant, factors and that the resulting changes in migratory activity are not compensated by subsequent increases or decreases of migratory activity. In normally day-active but nocturnally migrating birds, circannual signals cause alterations in the circadian system leading to the development of nocturnal 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 properties of the circadian system to Zeitgebers in such a way that circadian rhythms adjust faster to new conditions after long transmeridian flights.
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