The orientation system of birds - III. Migratory orientation

Many migratory birds show philopatry, i.e. they regularly breed and winter at the same sites. The routes taken by migrants are adjusted to the geograp hical and ecological conditions between breeding and wintering areas, often resulting in indirect paths. Young birds on their first migration face the task of reaching the as yet unknown population-specific winter quarters wi th the help of innate information. Large-scale displacement experiments wit h migrants and cage experiments with hand-raised birds revealed that this i nnate information is given as direction and distance, with the distance con trolled by an endogenous time program that determines amount and temporal d istribution of migratory activity. Both, migratory activity and direction - or, in the case of indirect routes, a sequence of directions - are genetic ally transmitted from one generation to the next. Birds use two reference systems to convert innate directional information i nto an actual trying course: celestial rotation and the geomagnetic field. Celestial rotation produces a reference direction opposite from its center, which is obtained by observing the diurnal and/or the nocturnal sky. This reference can be used to establish a star compass, not only utilizing the n atural, but also artificial "stars", provided the birds can observe these " stars" rotating. However, with only stars available, migrants that normally prefer southwesterly courses show southerly tendencies, apparently unable to convert the population-specific components of their migratory direction. Birds raised with only magnetic cues available, in contrast, are well orie nted in their population-specific migratory direction, except in areas with steep inclination; here, the magnetic field provides only an axis, and bir ds also need celestial rotation for unimodal orientation. As the birds' mag netic compass is an inclination compass, migrants of the northern and south ern hemisphere may use the same migratory program, starting out "equatorwar ds" in autumn. During the premigratory period, both reference systems interact to determin e the migratory course. If North indicated by celestial rotation and magnet ic North diverge, celestial rotation proves dominant, resulting in a change d magnetic compass course. However, celestial rotation does not simply over ride the magnetic course. In the natural situation, celestial rotation prov ides only the reference direction "opposite from the center of rotation", c orresponding to geographic South, which can be substituted by magnetic Sout h if birds have no access to celestial cues. Population-specific deviations from South seem to be coded only with respect to the magnetic field and ar e then added to the reference direction, resulting in the population-specif ic migratory course. These processes are interrupted if the sky is made to rotate in the reverse direction. The reasons for using two reference system s may lie in the fact that at higher latitudes, the magnetic field is stron gly affected by secular variations, while celestial rotation reliably provi des geographic South. At the same time, the magnetic field, being directly perceivable, may be better suited for indicating angular deviations. During migration itself, the relationship between the two reference systems changes, with the magnetic field becoming dominant. In case of conflict, c elestial cues are recalibrated according to magnetic North. The reasons for this shift in dominance may lie in celestial rotation ceasing to play a ro le. The sky changes its appearance as the birds progress, and the new stars are calibrated with the help of the geomagnetic field which becomes a reli able source of directional information at temperate and lower latitudes, Many birds change direction during migration. Their second compass course i s coded with respect to the magnetic field. The conversion of the respectiv e innate information appears to take place en route; a possible role of cel estial rotation has not yet been analysed. In Garden Warblers and Yellow-fa ced Honeyeaters, the shift in direction can take place under the control of the endogenous time program alone; Pied Flycatchers, in contrast, require magnetic conditions of the region where the shift normally takes place. At the magnetic equator, birds must reverse their course with respect to their magnetic compass from equatorwards to polewards in order to continue south wards. Here, the field of the equator with its horizontal field lines serve s as trigger. At the equator itself, where the magnetic compass becomes bim odal, birds may rely on celestial cues. The innate migratory program enables young birds to reach their general win tering area. The program becomes flexible at the end and allows them to loo k around for a suitable site to spend the winter. This becomes their winter "home" to which they return upon displacement. For the return migration to the breeding area and any later migrations, migratory birds can make use o f experience obtained during earlier travels. The migratory program still p rovides them with directional information: however, navigational processes based on site-specific information dominate over the innate mechanisms. Man y young birds undertake extended exploratory flights before they leave for migration, thus establishing a "map" of their future breeding area. As a co nsequence, they return to the normal breeding area after displacement. Adul t birds must be expected to choose their migration route by mechanisms of t rue navigation, thus avoiding unfavorable areas and revisiting good refueli ng sites, at the same time becoming less vulnerable to wind drift and simil ar phenomena. Details of these navigation processes are not known, as they have escaped experimental analysis so far. The dominant role of true navigation, which replaces the innate program, re presents a parallel to homing, where birds also rely on mechanisms of true navigation as soon as these become available.