Within Africa, the majority of Palaearctic landbird migrants depend on arthropods which are generally more abundant in the wet season, largely because the vegetation achieves most growth then (Morel 1973, Lack 1986b). Even in the dry season the Sahel zone is richer than it appears superficially, as trees and shrubs can be found at every stage of leaf, flower and fruit production. Locally and temporarily, insects can be available in tremendous abundance, as for example around standing water, where for periods at any time of year masses of chirono-mids coat the emergent vegetation. Periodic swarming of lakeflies (several species of chaoborid and chironomid midges) is well known at Lake Victoria. After larval life in the lake, enormous numbers of flies emerge in synchrony, usually around each new moon, forming huge clouds over the water. They often get blown ashore, forming an important food source for small insectivorous birds lasting for a few days at a time. These insects are exploited by migrants on both their northward and southward passage, enabling rapid fattening, but are not permanently available (Wanink & Goudswaard 2000).
Many of the passerines also eat fruit, particularly of Salvadora persica, Maerua crassifolia and Balarites aegyptiaca (Fry et al. 1970, Moreau 1972, Jones 1995, Stoate & Moreby 1995). The fruit crops vary greatly in size from year to year, but may remain available through the dry season, and are particularly favoured during the period of fat deposition for spring migration. Only the Common Quail Coturnix coturnix and Turtle Dove Streptopelia turtur are entirely granivorous, while the Short-toed Lark Calandrella cinerea, Ortolan Bunting Emberiza hortulana and Cretzchmar's Bunting E. caesia eat both seeds and arthropods during their stay in Africa.
Perhaps the most important element in the insect flush associated with the rains is the appearance everywhere of swarms of winged termites and winged ants. Because of their association with rain, they are in general available mainly to those migrants which extend south of the equator where the seasons are reversed. They are eaten by a wide range of birds which follow the rainbelts, from small warblers to large eagles. They are especially important to insectivorous falcons, and may account for the fact that four of the five migratory species winter mainly south of the equator. Hundreds of Lesser Spotted Eagles Aquila pomarina have been found at localities where winged termites are temporarily available (Brooke et al. 1972, Meyburg et al. 1995c).
On the other hand, the importance of plague locusts to migratory birds has probably been exaggerated (Moreau 1972). These insects erupt after several years of drought when the rains are good. They emerge immediately after rain, their eggs having survived in the ground for many years. Once the larvae have eaten the local vegetation, they swarm and head off downwind to low pressure areas where fresh rain is falling. However, in no part of Africa could locusts ever have provided a reliable source of food, and however superabundant they may be at certain times and places, years pass without outbreaks anywhere. In the intervals, the surviving insects merely take their place among many other solitary grasshoppers, some of which are very abundant. Nowadays, of course, locust outbreaks are often controlled by pesticide applications, which presumably remove many other arthropods too.
Another important food source for some raptors is provided by Quelea Quelea quelea, small seed-eating birds that nest in dense colonies, millions strong (Chapter 16). Colonies form after rain has fallen, so in the northern tropics they are available only for a few weeks after the migrants first arrive, but in the southern tropics, they are available for much of the austral summer, in one locality or another. These colonies attract hundreds of migratory raptors and storks which feed on the adults and chicks. Single colonies, occupying only a few hectares of scrub, have attracted more than 100 Steppe Eagles Aquila nipalensis or Lesser Spotted Eagles Aquila pomarina to localities where these eagles are otherwise seldom seen (Chapter 16). Many raptor species are also attracted to local concentrations of rodents, which occur sporadically, again mainly in response to local rain (Chapter 16).
Within habitats, Eurasian visitors tend to segregate, spatially or ecologically, from closely related African species (Moreau 1972, Thiollay 1989, Lack 1986b, 1987). The latter tend to be in different genera (so may be presumed to have somewhat different ecological requirements), and in arid areas are generally less abundant. In addition, resident waders are almost totally lacking in West Africa, leaving this niche largely free for the Eurasian migrants. In attempts to find how migrants coexist with residents, many researchers have compared the habitats, foraging behaviour and diets of Eurasian migrants with those of closely related African species (e.g. Lack 1986b, 1986, Rabol 1987, Jones et al. 1996, Baumann 2001, Salewski et al. 2002). Although these various studies have revealed some ecological differences between migrants and native species, this would be expected in comparisons of any two species, presumably reducing competition between them. Attention has focused on whether the Eurasian migrants have wider niches than the residents, exploiting a wider range of habitats and food sources. This seems true more often than not (Lack 1986b, Rabol 1993, Leisler 1992, Salewski et al. 2002), but depends on the species and area concerned (Salewski & Jones 2006).
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