Effects Of Droughts On Migrant Numbers

Water Freedom System

Survive Global Water Shortages

Get Instant Access

In much of Africa north of the equator, as emphasised above, rainfall during the northern summer largely determines the state of the vegetation and the extent of wetlands in the following dry season. It thus influences the food supplies of many birds, whether they consume plant or animal matter. But rainfall also varies greatly from year to year, and over much of the region since the late 1960s has in most years been well below former levels, owing to a failure of the rainbelts to extend so far to the north. Hence, drought conditions have been most severe along the northern edge of the Sahel zone, lying immediately south of the Sahara Desert, and have diminished southwards across the savannah zones towards the equator.

In the Sahel zone, rainfall deficits were particularly marked in 1968, 1973, and even more so in 1983 and 1984, and again in 1990. It was in 1969 (after the 1968 drought) that the importance of conditions in African wintering areas was first impressed upon European bird-watchers, when sudden and massive declines were apparent in the numbers of returning summer migrants, especially Greater Whitethroats Sylvia communis and Common Redstarts Phoenicurus phoenicurus, which are strongly represented in the northern Sahel (Winstanley et al. 1974). In Britain, according to the Common Bird Census of the British Trust for Ornithology, Greater Whitethroat numbers dropped by about 70% between 1968 and 1969 (Figure 24.4). Sand Martins Riparia riparia and Sedge Warblers Acrocephalus

Figure 24.4 (a) Rainfall trends in the Sahel region of Africa expressed as departure from the long-term mean (----), 1940-1988. Data from Grainger (1990). (b) Population trend (log scale) of Greater Whitethroats Sylvia communis breeding in Britain, as revealed by the Common Birds Census of the British Trust for Ornithology, 1962-1988. Data from Marchant et al. (1990).

Figure 24.4 (a) Rainfall trends in the Sahel region of Africa expressed as departure from the long-term mean (----), 1940-1988. Data from Grainger (1990). (b) Population trend (log scale) of Greater Whitethroats Sylvia communis breeding in Britain, as revealed by the Common Birds Census of the British Trust for Ornithology, 1962-1988. Data from Marchant et al. (1990).

schoenobaenus were also badly hit, even though most individuals spend only 4-6 weeks in the Sahel in October-November, before moving further south, and an even shorter period on return passage in March (Cowley 1979, Jones 1985). In Africa, as in Eurasia, both species are seen mainly around water bodies.

With the lapse of further years, it has become apparent that rainfall in the western Sahel has a major impact on the breeding populations of many species that nest in Europe, including some that overwinter in the Sahel and others that pass through. In a Europe-wide analysis of the population trends in breeding bird numbers, many European-African migrants showed sustained declines during 1970-2000, significantly more negative than those of short-distance migrants or residents (Sanderson et al. 2006). Analysis of the trends in 30 pairs of closely related species, one wintering in Africa and the other in Europe (e.g. Tree Pipit Anthus trivialis and Meadow Pipit A. pratensis, Garden Warbler Sylvia borin and Blackcap S. atricapilla), revealed significantly more negative trends in the former, regardless of breeding habitat. Further examination of species that winter in Africa revealed that those occupying dry open habitats declined significantly more than all those using other habitats. Some idea of the extent of population changes is given in Table 24.4, based on data from Britain. Also, the pattern of

Table 24.4 Population trend (%) during the latter part of the twentieth century of some species that breed in Britain and winter in Africa south of the Sahara

Species

Period (years)

Net change (%)

Osprey Pandion haliaetus

1973/77-1995/99

+ 658

Black-tailed Godwit Limosa limosa

1973/77-1995/99

Was this article helpful?

0 0

Post a comment