Other Possible Factors In Population Declines

Decreasing rainfall is not the only factor which may have caused declines in the numbers of some European birds that winter in West Africa. The burgeoning human population, and associated overgrazing, burning and woodcutting, together with increasing drainage and water abstraction, have all accentuated the process of desertification, so that even in relatively wet years many areas now hold little habitat for birds (Jones 1985, Thiollay 1989). Through these processes, desert is expanding at about 1% per year, with thousands of extra square kilometres added annually (Le Houerou & Gillet 1986). Indeed, some climat-ologists believe that, owing to human-induced destruction of vegetation, the climate of the Sahel zone has now flipped to an alternative steady state, represented by more desert-like conditions. Nearer the equator, more than 30% of African forest has been lost since 1970, and the cultivated land area has increased by 21% over the same period (UNEP 2002). Cultivation of rice and the irrigation of other crops has shrunk many wetlands used by migratory birds, reducing the surface area of Lake Chad, for example, from 25 000 km2 to 1350 km2 since 1960 (UNEP 2002).

Over much of the Sahel zone, the human population is now well above sustainable levels, and prevents any recovery of the vegetation and seed stock, which has proved possible locally, even with low rainfall, under total protection. Serious droughts have occurred in the past, as in 1911-1916 and 1940-1946, without any obvious lasting ecological effects. But since then the human and livestock populations have increased by more than 10-fold, leading to considerable impoverishment of the grasslands, disappearance of trees and shrubs, and shrinkage and degradation of wetlands. So even if rainfall returns to its earlier levels, some bird species may not regain their former abundance because of the habitat degradation that has occurred in the interim.

In addition, with the increasing use of pesticides, insects eaten by birds are likely to have become much less available, and many birds have themselves been killed directly by pesticide use (Mendelssohn & Paz 1977, Thiollay 1989, Mullie & Keith 1993). At least four Palaearctic species, which feed extensively on locusts and grasshoppers on their African wintering grounds, have declined in recent decades, including the White Stork Ciconia ciconia discussed above, Lesser Kestrel Falco naumanni, Montagu's Harrier Circus pygargus and Pallid Harrier Circus mac-rourus. However, in all these species, other factors may also have been involved (see below). On the other hand, the felling of rainforest in West Africa may have affected very few Eurasian migratory species adversely, because so few species winter there, and because the forest is replaced by more open secondary habitats ('derived savannah') which many species favour (Morel & Morel 1992). The same is true for miombo woodland in southern Africa (Ulfstrand & Alerstam 1977).

Not all Eurasian-African migrant species are necessarily limited primarily by factors acting on the wintering range, and for any one species the situation may differ from one part of the breeding-wintering range to another. Examples of species whose changes in breeding numbers have been linked with events in breeding areas include the Pied Flycatcher Ficedula hypoleuca for year-to-year changes (see Figure 26.4; Virolainen 1984), and the Turtle Dove Streptopelia turtur for long-term decline (Browne & Aebischer 2001). Other Palaearctic-African migrants whose declines have been linked at least partly to habitat changes in breeding areas include Corncrake Crex crex, Yellow Wagtail Motacilla flava, Lesser Kestrel Falco naumanni and Montagu's Harrier Circus pygargus (Norris 1947, Stowe et al. 1993, Donazur et al. 1993, Arroyo et al. 2002, Newton 2004b). In some species, as mentioned above, the relative importance of breeding and wintering conditions in influencing breeding numbers may well change from one period of years to another or from one region to another.

Hunting in the Mediterranean region was said to remove up to 1000 million birds each autumn, mainly migrant songbirds (Magnin 1991, McCulloch et al. 1992). This hunting is greatest in Italy, various Mediterranean islands (notably Cyprus and Malta), northern Africa and the Lebanon. The numbers of ringed birds reported each year from these areas are declining, but it is not known to what extent this reflects trends in populations, hunting or reporting rates. Nor is it known what effect this hunting has on breeding numbers. But because hunting occurs mainly in autumn, it could be offset to some extent by reductions in subsequent winter losses, if the total overwinter losses are density dependent (see Newton 1998b for evidence of winter density-dependence in several migratory species). In this case, autumn hunting would not have the large impact on population levels that might be thought from the numbers killed. This seems to be the case in Quail Coturnix coturnix, in which the numbers caught each autumn are related more to Sahel rainfall than to hunting pressure (Zuckerbrot et al. 1980), with numbers declining in recent decades (Tucker & Heath 1994). The numbers of Corncrakes Crex crex taken on the Mediterranean coast of Egypt in the autumns of 1993 and 1994 were estimated at about 9000 and 14 000 respectively, but these numbers formed only 0.5-2.7% of the total European breeding population (Baha el Din et al. 1996). There is also considerable hunting of migrants in Africa south of the Sahara. The emphasis seems to be on larger species, with many European Honey Buzzards Pernis apivorus killed in Liberia, White Storks Ciconia ciconia and Ospreys Pandion haliaetus in Nigeria, White Storks and Cranes Grus grus in Sudan, terns off West Africa, and many other species in these and other regions (Berthold 1993).

In some species, natural predation takes a considerable toll during migration. Two falcon species specialise on Eurasian-African migrants: Eleonora's Falcon Falco eleonorae which nests mainly on rocky islands in the Mediterranean and off the west coast of Morocco, and the Sooty Falcon F. concolor which nests along the coasts of the Red Sea and Arabian Gulf, and more sparsely in the deserts of Arabia, Israel, Egypt and Libya. Both species breed late, at the time of the autumn passage, and raise their young on migrants. Three other falcon species - the Peregrine Falcon F. peregrinus in the Mediterranean region, the Barbary Falcon F. pelegrinoides at scattered localities throughout the Sahara, and the Lanner Falcon F. biarmicus in the southern Sahara - are all resident year-round, but nest early, at the time of spring migration, and exploit the northward-bound migrants. Walter (1979) estimated that the world population of 10 000 Eleonora's Falcons F. eleonorae caught 1-2 million migrants annually, out of a total migration that Moreau (1972) considered to be in the order of 5000 million birds in the 1960s. Consequently the overall impact of the falcons' predation was assumed negligible (<0.1%). However, three species of shrikes (Lanius collurio, L. minor and L. senator) made up 15-20% of all birds taken (i.e. 200 000-400 000 shrikes per year). There is no reason to believe that the numbers removed by predation have increased during the twentieth century, however, when shrikes were declining over much of their European range. The same applies to the toll taken by the other falcons, which as yet cannot be estimated (Chapter 27).

Recently, additional predation on migratory birds in the Mediterranean region was found unexpectedly to involve the Giant Noctule Bat Nyctalus lasiopterus (Popa-Lisseanu et al. 2007). This mammal has a wingspan of 46 cm, and was first suspected of eating nocturnal migrants from the appearance of feather pieces in its droppings. More recently, blood samples taken from live bats over the period March-October showed that the isotope signatures of 15N and 13C changed seasonally in precisely the manner predicted if the bats shifted from a diet of insects in summer to birds in autumn. In spring, when migrant numbers were lower, signatures indicated a mixed diet of insects and birds. This bat hunts by aerial pursuit, and the virtual absence of birds from the diet in summer suggests a seasonal specialisation on passing migrants. Interesting though these findings are, the bats themselves are rare and localised, and it is unlikely that they have any appreciable impact on the collective migrant population.

Taking the evidence as a whole, it is hard to escape the conclusion that deteriorating conditions in Africa have been primarily responsible for the marked numerical declines of some Eurasian-African migrant species recorded in recent decades, and that these other potential limiting factors have been of most minor significance in this period. In other species, however, human-induced habitat changes in breeding areas may have caused or contributed to long-term population declines. Such evidence is available for at least seven species, about 4% of the Eurasian landbird species wintering in Africa, and further study may reveal more examples. In many species, the long-term declines may have been caused primarily by progressive land use and habitat changes in Europe, while year-to-year fluctuations about this trend have been associated with climatic and food conditions in Africa (for further discussion see Chapter 26). Moreover, although many Eurasian-Afrotropical migrants have declined during the last 50 years or more, some have increased - the Honey Buzzard Pernis apivorus and Hobby Falco subbuteo providing striking examples.

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