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Barn Swallow Hirundo rustica (1964-1998)

Britain

West Africa (Sahel)a +

Robinson et al. (2003)

aBirds winter in southern Africa, but pass through the Sahel in autumn and spring. The return journey falls at the end of the dry season.

aBirds winter in southern Africa, but pass through the Sahel in autumn and spring. The return journey falls at the end of the dry season.

breeding range but, as expected, the counts from some regions show different trends, and some show no evidence for links with Sahel droughts (Svensson 1985, Marchant 1992, Sokolov et al. 2001). Europe-wide trends would perhaps not be expected in species whose west European populations winter in West Africa, while east European populations winter in East or southern Africa, and are thereby exposed to different climatic regimes.

An analysis of long-term trapping data from a number of bird observatories scattered between Russia and Belgium (but mainly in eastern Europe) gave a somewhat different picture (Sokolov 2001). In many regions significant increases in numbers of passage passerines of many species occurred in the 1960s and 1980s, and significant declines in the 1970s and 1990s. In not a single species of 36 studied was a significant trend found for the overall period 1959-2000. Because in this area both short- and long-distance migrants adhered to the same broad fluctuations, the author attributed them to events in breeding areas, specifically 'long-term climatic fluctuations in the northern hemisphere'. In general, however, these populations were drawn from eastern, rather than western Europe, and among the tropical migrants, mainly wintered in East or southern Africa, as opposed to West Africa.

It is not just counts that point to the importance of wintering areas in influencing numbers of west European migrants. In key factor analyses, variation in overwinter loss was found to account for most of the variation in total annual loss in seven migrant passerine species monitored in Britain or Denmark (M0ller 1989, Baillie & Peach 1992). In addition, annual survival rates of Sedge Warblers Acrocephalus schoenobaenus and Greater Whitethroats Sylvia communis trapped each summer at specific sites in Britain were also correlated with rainfall in the Sahel zone (Figure 24.5; Peach et al. 1991, Baillie & Peach 1992), as were survival rates of Nightingales Luscinia megarhynchos at sites in Italy (Boano et al. 2004) and of Sand Martins Riparia riparia trapped at sites in Scotland and Hungary (Table 24.5; Bryant & Jones 1995, Szép 1995). Major crashes in Sand Martin numbers (or survival) occurred in 1968/69, 1983/84 and 1990/91, all following years of poor Sahel rainfall. Similarly, annual survival rates of Barn Swallows Hirundo rustica breeding in an area of Denmark were correlated with March rainfall in their southern African wintering areas (M0ller 1989). Although factors causing long-term trends in bird numbers are not necessarily the same as those causing annual fluctuations, they appear to be the same in these cases. These studies provided further circumstantial support for the proposed causal chain: low rainfall ^ low winter food supplies ^ lower overwinter survival ^ low breeding population.

It is not only insectivorous passerines that have shown such links. The numbers of Purple Herons Ardea purpurea nesting in the Netherlands over a 19-year period, and their annual survival rates, were correlated with wetland conditions in their West African wintering areas, as measured by water discharge through the Niger and Senegal Rivers (den Held 1981, Cavé 1983). The wetter the conditions in West Africa, the better was the overwinter survival and the greater the number of herons that returned to Holland to breed each year (Figure 24.6). A similar relationship was apparent among Night Herons Nycticorax nycticorax counted at colonies in southern France (Figure 24.6), and to a lesser extent among Squacco Herons Ardeola ralloides in the same localities (den Held 1981). A later study extended these findings to other colonies in western Europe, but not to colonies

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