4 to 150
5000 9000 13000
5000 9000 13000
Figure 24.6 Relationship between the numbers of migratory herons nesting at colonies in western Europe and wetland conditions in West Africa during the preceding year. Purple Herons Ardea purpurea were counted at one major colony in the Netherlands, and Night Herons Nycticorax nycticorax at several colonies in southern France. Wetland conditions measured as the sum of the maximum monthly discharges through the Senegal and Niger rivers. Both relationships were statistically significant (on Kendall's rank correlation test, P < 0.005). From den Held (1981).
further east in Europe (whose occupants are likely to have wintered further east in Africa) (Fasola et al. 2000). All these species have declined in western Europe since the 1960s, as has the Little Bittern Ixobrychus minutus which also winters in drought-prone areas of West Africa (Marion et al. 2000). In contrast, the Little Egret Egretta garzetta, which winters increasingly within Europe, has expanded over the same period, and its colony sizes showed no relationship with West African conditions (Fasola et al. 2000).
Similarly, the numbers of White Storks Ciconia ciconia that breed in western Europe have declined greatly since 1960, associated with a general decline in rainfall in their West African wintering areas. Annual variations in survival (measured from ringed birds) and subsequent reproduction (measured from nest studies) were closely linked to annual rainfall in the western Sahel zone, presumably because winter food supplies influenced the condition of adult storks in spring, and not only their survival (Bairlein & Henneberg 2000,2 Kanyamibwa et al. 1993, Barbraud et al. 1999). Moreover, following greater rainfall in the western Sahel from the mid-1980s, the west European population began to recover, bringing a 50% increase in breeding numbers between 1984 and 1994. The east European population, which winters mainly in East and southern Africa, having different rainfall regimes, showed a much less marked decline in the earlier period and a less marked increase in the later period (Dallinga & Schoenmakers
2The White Stork Ciconia ciconia in western Europe has also been affected by loss of habitat caused by drainage of wet pasture, which affected breeding success (Chapter 26). This may explain why populations in central Europe declined more rapidly than those in Spain, even though both populations winter in the same parts of West Africa.
1989, Schulz 1998). Differences in rainfall patterns may also explain the differences in population dynamics of some other conspecific populations of birds wintering in different parts of Africa, such as western and eastern populations of White Wagtail Motacilla alba (Svensson 1985). Overall, however, drought in African wintering areas has been implicated as a major causal factor in the regional population declines of at least 17 Eurasian breeding species over the period 1960-2000.
The importance of winter conditions is further shown by the fact that none of the passerine species mentioned above showed widespread declines in breeding rates over the periods concerned, or fluctuations in breeding rates that matched the subsequent fluctuations in breeding numbers. So far, however, the evidence that conditions on the wintering areas influence the numbers of breeding birds is based almost entirely on correlations between annual rainfall, water levels or river flow on the one hand and year-to-year changes in population or survival on the other. Improvements in some correlative analyses might be made in future if the wintering areas of particular sectors of breeding populations could be defined more precisely, and if weather data were then chosen to match those areas more accurately. Greater understanding of mechanisms would probably emerge if more field studies were conducted in Africa itself, where food supplies could be measured. The clear implication of current findings, however, is that climatic conditions up to several thousand kilometres south of the breeding areas affect the spring population levels of some species of trans-Saharan migrants that breed in Europe.
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