Some of the earliest studies in the field involved trapping wild birds on autumn migration, transporting and releasing them in a distant location, and using the resulting ring recoveries to find where they went (Figure 9.1). Such simple but large-scale experiments revealed a fundamental difference in behaviour between experienced adult and naive young birds. Whereas the older birds were able to reach their traditional winter areas, even from sites outside their normal migration route, young birds on their first autumn migration proved unable to do so. Instead, they continued on their usual migratory heading for about the same distance they would normally travel from the capture site. The inference was that inexperienced young migrants are guided on migration by innate information expressed as a direction and distance from the starting point, distance being controlled by the duration of migratory activity (Chapters 12 and 20). The innate information would be equivalent to an instruction like: 'travel for six weeks towards the southwest', or, in cases of non-straight routes: 'travel for six weeks toward the southwest and then for five weeks toward the south-southeast' (Wiltschko & Wiltschko 2003). This system is known variously as clock-and-compass, bearing-and-distance or vector migration. On the other hand, experienced migrants that have travelled the route before can make use of their experience, as well as innate information, on subsequent migrations. They reveal an ability to navigate, that is to head towards a specific point on the earth's surface from some distant location. Homing to a known site is more complicated than the ability to head only in particular compass directions because it involves true bi-coordinate navigation, requiring a map sense (also known as goal orientation).
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