As mentioned above, Lesser Whitethroats Sylvia curruca showed different directional preferences under planetarium skies representing different points on their migration route (Sauer 1957). Other experiments using an artificial magnetic field implied that some birds use magnetic information to indicate regions where they must stop migrating, change direction or accumulate large fat reserves before crossing a barrier. Adult Tasmanian Silvereyes Zosterops lateralis were tested near the mid-point of their south-north migration in southeast Australia. Birds exposed in captivity to artificially-generated magnetic field values of inclination and intensity normally experienced near the start of their migration, oriented north-northeast. In contrast, birds exposed to magnetic field values that they would experience near the end of their migration ceased to show any significant directional preference; in this respect, they acted as though they had arrived in wintering areas (Fischer et al. 2003). These findings thus pointed to the involvement of the geomagnetic field in the migration of adult Silvereyes. In contrast, no effects of changing the artificial magnetic field were noted on inexperienced young birds caught prior to their first migration.
Young migratory European Pied Flycatchers Ficedula hypoleuca from western Europe showed a distinct change in compass heading when exposed in captivity to values of the magnetic field normally encountered in southern Europe, where the normal migratory route shifts from southwest to south. The altered magnetic field was followed by the shift in orientation only when applied at the appropriate time during the migratory period (Beck & Wiltschko 1988). In Pied Flycatchers, therefore, the magnetic conditions of the location where the change is to occur and the time programme evidently interact to produce an appropriate response at an appropriate latitude. As mentioned above, magnetic inclination gives a good indication of latitude, and could thus trigger change in direction or fattening (Wiltschko & Wiltschko 1995).
Some juvenile Thrush Nightingales Luscinia luscinia, caught in autumn in Sweden and exposed there to the geomagnetic conditions they would normally experience in northern Egypt, accumulated high fat levels appropriate to the subsequent desert crossing. They contrasted with control birds, exposed to local geomagnetic conditions, which accumulated much smaller fuel loads typical for south Sweden (Kullberg et al. 2003). However, birds trapped late in the onset period of autumn migration accumulated a high fuel load irrespective of magnetic treatment. It seemed that the relative importance of endogenous and environmental factors in individual birds was affected by time of season, as well as by geographical area (see also Chapter 12).
These various experiments indicate that inexperienced birds on their first migration can detect and make use of the geomagnetic field, at least to indicate when major changes are needed during their journeys. The implication is that such birds have an inborn response to external geographic cues (especially geomagnetic cues) that are characteristic of certain latitudes or regions, and that they can use particular conditions to trigger a halt to migration, or a change in direction or fattening regime (Beck & Wiltschko 1982, 1988, Fransson et al. 2001).
We thus have two potential mechanisms which could act in these ways. The first is an endogenous time programme which switches particular activities on and off at appropriate times in the migration cycle (Chapter 11). The second is a response to particular latitudes or more specific regions, at least partly through regional magnetic or night sky conditions, which can similarly trigger appropriate changes in migratory behaviour. How much these separate mechanisms act independently or in conjunction with one another is an open question, but different species would not necessarily be expected to respond to particular experiments in the same way.
The concentrations of ring recoveries of some bird populations in areas north of the Sahara Desert in autumn indicate that individuals from these populations favour these areas to fatten before their desert crossing (Chapter 5). If birds were to use magnetic information as a cue to where to fatten, they must be able to distinguish the small differences in magnetic field between such areas. A long-distance migrant of the New World, the Bobolink Dolichonyx oryzivorus, has been shown to detect changes of 200 nT (nanotesla) electrophysiologically (Semm & Beason 1990), and studies on White-crowned Sparrows Zonotrichia leucophrys gambelii indicate that the magneto-receptors are extremely sensitive to small changes (less than 3°) in the angle of geomagnetic inclination (Akesson et al. 2001). Differences in the total intensity and inclination of the field between the centres of different major stopover areas for trans-Saharan migrants taking the eastern Mediterranean route into Africa (Libya-Egypt, Cyprus, and Israel-Jordan-Syria-Turkey) are within these limits, indicating that geomagnetic cues could be relevant for populations migrating to these population-specific fuelling areas (Fransson et al. 2005). Studies on other kinds of animals have reported sensitivities to the magnetic field as small as 120-50 nT, so once again birds seem not unusual in their magnetic sensitivity. If young birds are responding in the way that ringing and experimental findings suggest, they are not just using a clock-and-compass system on their first migration, but are also benefiting from an inborn response to magnetic cues, which leads them to converge on specific regions.
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