: Mouritsen and Larsen 1998). This led to the conclusion that inexperienced young migrants are guided by innate information on their migration route

I.- S Mechanisms of Orientation and Navigation in Migratory Birds ~oswitha Wiltschko and Wolfgang Wiltschko' 1 The Task of Reaching Distant Winterquarters Every year in autumn, myriads of migrating birds
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I.- S Mechanisms of Orientation and Navigation in Migratory Birds ~oswitha Wiltschko and Wolfgang Wiltschko' 1 The Task of Reaching Distant Winterquarters Every year in autumn, myriads of migrating birds leave their breeding areas and start the long flight towards their wintering areas, which often are thousands of kilometres away. Migration means several weeks or even months of travelling, which may include crossing ecological barrieres, like mountains, seas and deserts. The birds have to face and solve a number of physiological and ecological problems, like meeting the energy requirements of the extended flight and finding suitable refuelling areas along the route etc. A major problem, however, concerns orientation and navigation: while adult mi-,-. grants are already familiar with the route and a winter quarter that allowed their survival during the previous year, the young birds migrating for the first time have to reach their population-specific wintering area that is still unknown to them. To find their way to B distant site, birds need a reference system, a factor that is available to them in the breeding area, en route and also in the wintering area. Two types of factors meet this requirement: the geomagnetic field and celestial cues, and both are used by birds as compass mechanisms. In migrants, the magnetic compass (for summary, see R. Wiltschko and Wiltschko 1995), the star compass (Emlen 1967a,b) and a mechanism based on sunset cues (Able 1982; Moore 1982) have been analyzed in some detail. A crucial question, however, involves the basic requirement of any compass use: how do migrating birds know what course to take? Large-scale displacement experiments with migrants (e.g. Drost 1938; Perdeck 1958, 1964) revealed a fundamental difference in navigational procedures between adult and young birds (Fig. 1): while the older birds were able to determine the course leading to their traditional wintering area even from sites outside their normal migration route, young birds on their first migration proved unable to do so. Instead, they continued with their normal migratory heading for about the distance they would normally travel (see also : Mouritsen and Larsen 1998). This led to the conclusion that inexperienced young migrants are guided by innate information on their migration route Fachbereich Biologie und Informatik der J.W.Goethe-Universitat, Zoologie, Siesmayerstrasse 70,60054 Frankfurt a.m., Germany, P. Berthold, E. Gwinner, E. Sonnenschein (Eds.) Avian Springer-Verlag Berlin Heidelberg 2003 434 Roswitha Wiltschko and Wolfgang Wiltschko given in polar coordinates, namely as a distance and a direction from the starting point (see also Mouritson 1998,2000). An endogenous program that is genetically transmitted from one generation to the next (e.g. Berthold and Querner 1981; Helbig 1991; for summary, see Berthold 1999) determines the direction and the approximate length of the route, the latter by controlling Fig. 1. Displacement experiments with starlings during autumn migration. Birds of Baltic origin were caught as transmigrants in The Netherlands at site F near Den Haag, displaced to Switzerland and released at Basel (R,), Zurich (R,) and Genf (R,). The symbols mark the sites of ringing recoveries during autumn and winter following displacement: open, adult migrants; solid, juveniles on their first migration. (After Perdeck 1958) - - Mechanisms of Orientation and Navigation in Migratory Birds 435 the duration of migratory activity (e.g. Gwinner 1968, 1974). Put into human terms, the innate information would be equivalent to an instruction like: fly for 6 weeks towards southwest , or, in cases of non-straight routes like e.g. those of garden warblers, Sylvia borin, from the southern German population (Fig. 2), to something like: fly for 6 weeks towards southwest and then for seven weeks towards south-southeast . Fig. 2. The migration route of garden warblers of the southern German population as indicated by the spontaneous directional preferences of hand-raised birds. The arrows indicate the mean headings at the dates given, with the respective arrow originating in the area through which garden warblers would normally pass at the indicated date. Hatched areas: widely hatched, breeding area; narrowly hatched, wintering area. (After Gwinner and Wiltschko 1978) 436 Roswitha Wilwhko and Wolfgang Wiltschko For migrants relying on innate information, migratory orientation may be considered a two-step process, analogous to the navigational process in homing (Kramer 1959; for a detailed discussion, see R. Wiltschko and Wiltschko 1999a): in a first step, they must derive a compass course from the innate information on direction, e.g. obtain a specification equivalent to a term like southwest or 225O , and in a second step, they must locate this course with the help of a compass, thus transforming it into a specification of the type this way or go there . The two steps correspond to two consecutive phases in migratory orientation, with the first phase taking place mainly during the premigratory period, the second phase during the first migration itself. Experienced migrants, on the other hand, are familiar with the ecological situation and navigational factors en route and at the goal area; they may be expected to utilize this information when starting a new journey and may directly head towards their wintering or breeding area. The orientation processes guiding young migrants on their first migration have been extensively analyzed in numerous orientation experiments of caged migrants. Here, the mechanisms used and their interactions are fairly well known. This contrasts with the rather limited knowledge on mechanisms used by experienced migrants on their second and later migrations, where the experimental analysis has so far been mostly devoted to compass orientation only. Night-migrating birds are by far the best-studied group, because their migration-motivated nocturnal activity, Zugunruhe, is easily separated from other activity. In the present chapter, we will review the existing evidence and outline how birds orient their flights during their extended travels. 2 Establishing the Migratory Direction as a Compass Course The first task that first-year migrants face is converting the genetically coded information on their migratory direction into an appropriate compass course. This has to take place during the premigratory period so that at the end of this phase, when the bird is ready to take off, the migratory course is available. For technical reasons, the processes establishing the migratory course can only be analyzed by studying their after-effects during migration. Hence, the normal procedure was to hand-raise nestlings of migrants, among them indigo buntings, Passerina cyanea, garden warblers, blackcaps, Sylvia micapilla, pied flycatchers, Ficedula hypoleuca, and savannah sparrows, Passer- CU~US sandwichensis, while limiting and/or manipulating their premigratory experience. Later tests during the migration season revealed whether these birds had been able to properly establish a migratory direction under the set circumstances and what direction it was. -. Mechanisms of Orientation and Navigation in Migratory Birds Two Reference Systems Experiments have shown that migratory birds make use of two references when converting innate directional information into an actual direction: celestial rotation indicating geographic north and the geomagnetic field indicating magnetic north. Birds can perceive the course of the magnetic field lines with the help of their magnetic compass sense so that magnetic north is directly accessible to them. Celestial rotation, in contrast, must be derived from observing the apparent motion of celestial objects, either the stars at night (Emlen 1970) or the changing pattern of polarized light during the day (Able and Able 1993, 1995a; Weindler et al. 1998). The role of the geomagnetic field as reference for the migratory direction was demonstrated by experiments with several species of young passerines. Birds were hand-raised without access to celestial cues and, when tested during autumn migration, headed in their species-specific migratory direction with the magnetic field as only cue (Fig. 3 left; e.g. W. Wiltschko and Gwinner 1974; Bingman 1981; Beck and Wiltschko 1982; Bletz et al. 1996). These findings indicated that the geomagnetic field alone is sufficient for establishing the migratory course, at least at temperate latitudes. At higher latitudes, however, where the angle of inclination is steeper, the situation appeared to be somewhat different. Blackcaps and pied flycatchers preferred their migratory direction and the reverse direction - the magnetic field alone seemed to indicate merely the migratory axis. The birds changed to a unimodal preference of their migratory direction only after they had additionally observed celestial rotation (Shumakov and Zelenova 1988; Weindler et al. 1995). During the premigratory period: rotating 'stars' together with magnetic field only magnetic information rotating 'stars' only Fig. 3. Orientation behavior of hand-raised garden warblers that were exposed to different combinations of magnetic and stellar cues during the premigratory period and during testing; orientation recorded during the first leg of migration. Conditions during testing are given within the circle; those during the premigratory period above. The symbols at the periphery of the circle indicate headings of single tests (left) or mean headings of individual test birds (centre and right); the arrows represent the resulting mean vectors. N = north, indicated by the stars; mn = magnetic north. The two inner circles mark the 5% (dotted) and the 1% significance border of the Rayleigh test. (Data from W. Wiltschko et al and Weindler et al. 1996) 438 Roswitha Wiltschko and Wolfgang Wiltschko The role of celestial rotation for establishing the migratory course was first demonstrated by Ernlen (1970). He hand-raised young indigo buntings and exposed them to a planetarium sky rotating around Beteigeuze in the constellation Orion instead of the Polar Star Polaris. Tested under the same, now stationary planetarium sky during autumn migration, the birds headed away from Beteigeuze - they obviously interpreted the former centre of rotation as geographic north and headed away from it, i.e. southward . This suggested that migrants learn to interpret the star patterns by observing celestial rotation. The importance of celestial rotation as crucial cue is also indicated by experiments in which, during the premigratory period, the complex starry sky was replaced by a simple pattern of just 16 small lights that rotated with one rotation per day: tested during autumn migration, the birds headed away from the direction that had been the centre of rotation (Fig. 3 centre, right; W. Wiltschko et al. 1987; Weindler et al. 1996), while control birds that had been exposed to the same artificial sky , yet without seeing it rotate, were disoriented (W. Wiltschko et al. 1987). These findings emphasize the fundamental role of celestial rotation during the premigratory period: apparently, birds have no innate knowledge of the starry sky, but head away from the centre of any sky as long as they could observe it rotating around this centre - the number of stars and the form of the pattern itself are irrelevant. Later studies showed that the diurnal sky also conveys information on celestial rotation (Able and Able 1993; Weindler et al. 1998). Interestingly, not the sun moving across the sky, but the changing pattern of polarized light during the day proved crucial (Able and Able 1993). Even the polarization pattern around sunset and sunrise alone was sufficient (Able and Able 1995a). Information on celestial rotation is thus available to the young migrants during the day as well as during the night. 2.2 Interactions Between Celestial Rotation and the Geomagnetic Field The findings described so far seem to suggest that the geomagnetic field and celestial rotation represent two independent reference systems that both guarantee the establishment of the appropriate migratory course. However, birds like southern German garden warblers, whose population-specific migratory direction deviates markedly from due south (see Fig. 2), appear to need magnetic information to establish their normal southwesterly course. While birds hand-raised and tested with the geomagnetic field as only cue headed southwest (Fig. 3 left; Wiltschko and Gwinner 1974), the orientation of birds that had been exposed to a rotating artificial sky and were tested under the same sky without magnetic information depended on their experience during the premigratory period: birds that had observed the rotating sky in the local geomagnetic field preferred their population-specific southwesterly direction (Fig. 3 centre), whereas those that had experienced the '0- tating sky in a compensated magnetic field headed due south (Fig. 3 right; Weindler et al. 1996). These findings suggest that celestial rotation alone, in Mechanisms of Orientation and Navigation in Migratory Birds 439 contrast to the geomagnetic field, does not allow the birds to convert their innate information completely - it provides the birds with a direction away from its centre only. This direction, corresponding to geographic south, has to be combined with information from the geomagnetic field to establish the population-specific southwesterly migratory direction (Weindler et al. 1996). This leads to the general question about interactions between the two reference systems. Experiments with hand-raised birds that were exposed to abnormally large differences between the centre of celestial rotation and magnetic north during the premigratory period indicated a dominance of celestial rotation over the geomagnetic field. When young birds that had been raised under the natural sky in a deflected magnetic field were tested with the magnetic field as only cue, they preferred the magnetic direction that had been pointing towards their natural migratory direction during the exposure to the sky - their headings were shifted with respect to their controls by the same amount, but in the opposite direction as magnetic north had been deflected (Fig. 4; Bingman 1983a; Able and Able 1990a,b, 1993, 1995a,b; Prinz and Wiltschko 1992). Obviously, celestial rotation had altered the magnetic compass course. Interestingly, these birds did not prefer the magnetic direction that had been opposite to the centre of celestial rotation, but that which had been in their true migratory direction deviating from due south. The reverse experiments with birds of similar premigratory experience tested with stars as only cues clearly showed that an abnormal directional relationship between magnetic and celestial cues does not affect the orientation with respect to celestial cues (Bingman 1984; W. Wiltschko et al. 1987). For more details on the interaction of celestial and magnetic cues during the premigratory period, see W. Wiltschko et al. (1998) and R. Wiltschko and Wiltschko (1999b). Fig. 4. Orientation behaviour of pied flycatchers that were exposed to the natural sky in a magnetic field whose north was shifted to 240 WSW during the premigratory period; tests with the local geomagnetic field as only cue. [mn] position of magnetic north during the premigratory period. The symbols at the periphery of the circle indicate headings of single tests; other symbols as in Fig. 3. (Data from Prinz and Wiltschko 1992) 440 Roswitha Wiltschko and Wolfgang Willschko 2.3 Forming the Migratory Course from Two Components At the first glimpse, the findings about the interaction between the two reference systems might appear somewhat confusing - information from celestial rotation overrules that from the geomagnetic field and changes the magnetic compass course, while, at the same time, magnetic information is crucial for the development of the population-specific migratory course, at least in the only species where this aspect has been analyzed so far. However, if we look upon the innate directional information as consisting of two components (Fig. 5), these findings can be reconciled (see Weindler et al. 1996): one component would be a direction that provides a basic reference for the migratory direction, which can be provided by both celestial rotation and the geomagnetic field, corresponding to geographic and magnetic south, respectively. Birds normally prefer celestial rotation, as the dominance of celestial rotation suggests (e.g. Bingman 1983a; Able and Able 1990a,b; Prinz and Wiltschko 1992). If celestial rotation is not available, however, birds can use magnetic south instead, as indicated by the normal orientation of birds that were hand-raised without view to the sky (e.g. W. Wiltschko and Gwinner 1974; Beck and Wiltschko 1982; Bletz et al. 1996). The population-specific deviation directional infomation I celestial rotation ( I geomagnetic field I reference direction population-specific deviation fromthe reference direction Fig. 5. Interaction between celestial rotation and the geomagnetic field in the processes converting the innate directional information into a compass course. (After W. Wiltschko et al. 1998) Mechanisms of Orientation and Navigation in Migratory Birds 44 1 from this direction, on the other hand, appears to be coded only as a magnetic angle, e.g. in the case of the southern German garden warblers, as something similar to 45' clockwise from the reference direction . Both components must be combined to form the population-specific migratory course (see Fig. 5). This complex way of coding the migration course may seem surprising at first, but it offers considerable advantages to the birds. Celestial rotation inevitably indicates geographic south. By using it as reference for the innate directional information, it is guaranteed that this information is converted into a course that always corresponds to a specific geographic direction. This is crucial because when travelling over hundreds or thousands of kilometres, even a small deviation from the correct course may add up to a considerable mistake and lead the birds into regions outside their population-specific wintering area, where survival might be difficult. Magnetic south, in contrast, varies over longer time intervals because of the secular variation of the geomagnetic field (see Gaibar-Puertas 1953) - magnetic declination (the difference between geographic and magnetic north) at any given site undergoes considerable changes. Hence, celestial rotation provides a much more suitable reference for information that is genetically passed from one generation to the next over the years. Relying on celestial rotation alone, however, might cause other problems. Unlike magnetic north, the reference direction provided by celestial rotation cannot be directly perceived; it is a mental construct based on long-term observation of the rotating sky. In view of this, it might not be well suited for coding angular deviations. Also, if the critical period of establishing the migration course coincides with a period of heavy cloud cover and overcast, the birds might have only insufficient access to the cues indicating celestial rotation. Here, the magnetic field provides a helpful backup system. On the other hand, while the migratory directions of long-distance migrants like garden warblers will always include a strong southerly component, the specific starting course varies between populations. More important, it will also vary in the same population with time, as the ecological situation en route and in the wintering area changes as a result of climatic changes. Hence the populationspecific angular deviation from geographic south is the component of the migratory direction that is under a constant selective pressure to optimize the migration
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