Building Your Own Outdoor Aviary
Much has been learned about the proximate control of migration from studies on captive birds. Under natural daylengths, caged birds from obligate migratory populations develop fat reserves and migratory restlessness at appropriate dates in autumn and spring, at about the same times as their wild counterparts. Evidently, the same factors that stimulate departure in wild birds trigger restlessness in captive ones kept on natural daylengths (Gwinner 1972, Berthold 1996).
In general, the longer the distance between breeding and wintering areas, the greater the duration and intensity of migratory restlessness shown by caged birds (Box 12.1). Different Sylvia warblers migrate average distances varying from a few hundred to nearly 6000 km, and show corresponding average periods When caged birds reach a migratory state, they usually show periods of wing-whirring, together with increased perch-hopping, body-turning and other activities. Cages are too small to allow flight and, while birds that are caged straight from the wild flutter around and try to escape, they eventually adjust to cage life, and show wing-whirring instead - a cage-adapted behaviour, viewed as 'migration in sitting position' (Berthold 1996). Migratory restlessness (or Zugunruhe) has now been recorded in caged birds from more than 100 different species. It can be quantified from the alteration of the normal pattern of diurnal locomotor activity, especially the increase of activity peaks...
In recent years, the role of genetic factors in the control of migration has been shown experimentally, mainly by Peter Berthold and his colleagues, who found that Blackcaps Sylvia atricapilla and other songbirds could be bred on a large scale in aviaries (Berthold & Helbig 1992, Berthold 1995, 1999). Different populations of Blackcaps varied from completely migratory to completely sedentary in different parts of the range. The various migratory populations also differed in their migration dates and in the directions and distances travelled. Most of these traits could be measured in captive birds by assessing the timings and amounts of migratory restlessness (or Zugunruhe), a specific behaviour involving fluttering and wing-whirring which appears in caged birds mainly at migration seasons (see Box 12.1, Figure 20.2). Directional preferences of individuals could be assessed by placing them in orientation cages, and checking where they most frequently headed. Both restlessness and...
Another major benefit of studying migratory orientation in caged birds is that the external information received by the bird can be manipulated. For example, the perceived position of the sun can be altered by use of mirrors, star patterns can be modified in a planetarium, or the geomagnetic field can be altered using large magnetic coils (Wiltschko & Wiltschko 1995). These procedures facilitate study of the external cues that might be used by birds to determine their migratory direction (Chapter 9).
During reintroduction projects at various localities in western Europe, White Storks Ciconia ciconia were reared and kept in aviaries for at least one winter, which prevented them from migrating as they normally would. After they had been released, they remained in the same areas year-round, breeding and wintering there, and supported by supplementary food in winter. However, the free-living offspring of these birds migrated as normal for their population, yielding ring recoveries along the usual southwestern migration route (Fiedler 2003). These findings provided further indication that experience based on learning and memory can modify the inherent migratory behaviour of individuals. The findings from these different types of experiment on the role of experience on migration behaviour are summarised in Table 12.1.
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