Official journal of the Netherlands Ornithologists' Union
|Zwarts L., Bijlsma R.G. & van der Kamp J. (2023) The fortunes of migratory birds from Eurasia: being on a tightrope in the Sahel. ARDEA 111 (1): 397-438
|Many studies have shown that rainfall in the Sahel has a great influence on population trends of European bird species that spend the northern winter there. African bird species living in the Sahel, notably those that forage on the ground, have also shown significant declines, but independent of rainfall. This paper summarises the results of field data gathered in the entire Sahel and evaluates the many factors that play a role in the fortunes of birds. (1) Rainfall determines the extent of open water in the Sahel, and by default the fortunes of waterbirds. In recent decades the surface area of open water has increased because water tables have risen. (2) Rainfall south of the Sahel determines river discharge and therefore the surface of floodplains in the Sahel. Rainfall has a cumulative effect: discharges disproportionally decrease after a number of years with little rain, and vice versa. During the dry season (October–May), floodplains gradually dry out. In wet years, water – and hence food – is available for birds up to their departure, but in dry years birds become concentrated at the few remaining pools and so present an easy target for bird-trappers. Further desiccation leads to starvation. (3) After a year with heavy rainfall, seed is available in abundance, but a dry year results in a shift in the plant community and a low seed supply. Mortality among seedeaters increases under dry conditions. (4) In dry years, trees lose their leaves early on, forcing arboreal birds into a diminishing number of trees that retain leaves. In extremely dry years trees die on a massive scale and it takes many years before tree coverage is restored. When droughts occur in quick succession, as in 1972/73 and again in 1984/85, tree recovery is slow and populations of arboreal birds will continue to decline, or recover slowly or only partly (as for Eurasian Wryneck Jynx torquilla and Common Redstart Phoenicurus phoenicurus, whose numbers remain reduced by tenfold when compared to the 1950s, despite a slight recovery). Rainfall in the Sahel gradually recovered after 1990, as did the woody vegetation albeit with a delay, and many migratory bird species responded accordingly. Subalpine Warblers Curruca subalpina and Western Orphean Warblers Curruca hortensis have increased as much as threefold to fivefold since 1990. Southern European bird species, wintering in the arid parts of southern Sahara and Sahel, were hit the hardest during the Great Drought in 1969–1992, but also recovered the fastest, particularly strongly once rainfall had significantly recovered. Despite clear links between migratory birds and rainfall-related variables in their wintering areas, a migrant’s world is more complicated than exclusively being constrained by rainfall. In the past century, the human population in sub- Saharan Africa has increased tenfold, with far-reaching consequences. (1) Cattle numbers boomed and grazing pressure increased greatly. Heavy grazing means lower grass seed production, especially of seeds that birds prefer. The steep decline of granivorous birds over the past several decades is therefore no surprise. (2) Every year, 2% of the savannah is converted into farmland. This is unfavourable for most, but not all, bird species. Farmers selectively favour particular tree species on their land (and remove the rest), by which the resulting tree species composition differs from the typical variety found in savannah. The shift in tree composition particularly disfavours birds wintering in both the more arid and more humid vegetation zones. Arboreal birds wintering in the intermediate zone have benefitted from the expansion of farming, because their preferred tree (White Thorn Faidherbia albida) is favoured by farmers and has become more abundant. (3) The expansion of agriculture is most obvious in the low-lying, slightly more humid and more fertile soils, much to the detriment of flooded forests, which being important refuges during droughts are particularly rich in birds. Most flooded forests have disappeared from the Sahel. (4) Millions of hectares of humid woody savannah have been converted into Cashew Anacardium occidentale plantations since 1980. Cashew plantations are almost completely devoid of birds and so constitute a significant loss of a once highly diverse wooded habitat. (5) The proliferation of Prosopis juliflora, an exotic tree species resembling an acacia, has locally and regionally displaced indigenous acacia species. Prosopis attracts few birds in comparison with acacias. (6) Irrigation and dam construction have decimated the size of floodplains in Senegal, Mali, Nigeria and Chad, with corresponding effects on many bird species, especially waterbirds, that concentrate in these areas. (7) Massive bird catches, such as those in the Inner Niger Delta after about 1990, did not occur in the past when cheap nylon nets (initially used for fishing) were not yet available. Furthermore, storage and transportation of trapped birds were impossible until ice and vehicles became available. (8) Large bird species have virtually disappeared from the Sahel due to human predation, especially in the densely populated western part. As a wintering area for most migratory Eurasian birds, the Sahel has become less attractive. Far fewer migratory birds now use the Sahel than half a century ago, not least because many populations have substantially declined in numbers. Furthermore, several European migrant species increasingly are wintering north of the Sahara, which may relate to impoverished conditions in the Sahel, but as likely to significant habitat changes in Europe and the pace of climate change.