Movements Within Africa

The Sahel savannah zone that crosses the continent from west to east is particularly important for the migrants. Not only is it the first area with food that they reach after crossing the Sahara in autumn and the last before crossing in spring, but many species spend the whole or part of the winter there (Moreau 1972, Jones 1999). The migrants arrive between August and November, depending on species, but mainly in September when many African birds (which have spent their breeding season there) are themselves beginning to retreat southward. At this time the rains are finishing, the land is green and food is plentiful, but as the country gradually dries out, feeding conditions deteriorate. Nevertheless, many of the dominant trees have leaves (supporting insects) or produce flowers and fruit in the dry season, which many birds can use. About one-quarter of all migrant species remain in the Sahel and Sudan zones for the whole winter in the local dry season (strategy 1 in Table 24.3). Examples include some Sylvia warblers, which eat fruit as well as insects from leaves, and some ground-dwelling wheatears and chats which eat small terrestrial insects (Pearson & Lack 1992). They also include granivores, for seeds remain available on the ground throughout the dry season, but disappear rapidly when rain causes their germination. It would make no sense for a wintering seed-eater to move south of the equator in September to the austral summer.

Many other migrant species stay in the Sahel-Sudan zones for only 1-2 months; some moult while they are there, and then move on further south, in late October-November (strategy 2 in Table 24.3). Such species are conspicuous in both Sahel and Sudan zones for 4-6 weeks at the end of the rains, and then disappear suddenly with the first arrival of the dry and dust-laden 'Harmattan' wind from the north (Jones 1985, 1995, 1999). In West Africa, most such species undertake a second migration of up to several hundred kilometres to pass the rest of their stay in the less arid Guinea and derived savannahs to the south (their movements being limited to at most a 12° latitudinal span by the Gulf of Guinea coast or the central African rainforest. However, other species, after spending 1-2 months in the Sahel-Sudan zones, perform a much longer second migration to cross the equatorial forest, and spend the rest of their stay in the wet season conditions of the southern African woodlands and savannahs (strategy 3 in Table 24.3). Examples include the Sand Martin Riparia riparia that feeds on aerial insects, and various warblers that glean small insects from fresh leaves.

Relatively few species seem to fly directly to winter quarters in southern Africa, passing quickly through the northern tropics (strategy 4 in Table 24.3). They include the Barn Swallow Hirundo rustica, Spotted Flycatcher Muscícapa striata and Common Cuckoo Cuculus canorus. These species tend to leave Europe relatively early, and pass through the northern tropics while it is still raining, reaching the southern tropics as the rains begin. They thus spend the maximum time possible in wet season conditions. These species also feed mainly on the flush of insects which appear on new leaves, or on the aerial and ground insects brought out by rain.

Table 24.3 Migratory habits of some Palaearctic migrants in Africa. Some species occur in more than one category, usually where migration patterns differ between the west and east sides of Africa

Strategy 1: Wintering entirely in the dry season conditions of the northern Sahel

Strategy 2: Wintering in dry season conditions, first in the northern Sahel-Sudan savannahs and then further south in the Guinea and derived savannahs

Strategy 3: Wintering in wet season conditions, first in the northern tropics, and then in the southern tropics (transequatorial migrants)

Strategy 4: Wintering entirely in wet season conditions in the southern tropics, after passing rapidly through the northern tropics (trans-equatorial migrants)

Greater Whitethroat Sylvia communis, Lesser Whitethroat Sylvia curruca, Northern Wheatear and Sudan savannahs Oenanthe oenanthe, Olivaceous Warbler Hippolais pallida, Bonelli's Warbler Phylloscopus bonelli, Subalpine Warbler Sylvia cantillans, Orphean Warbler Sylvia hortensis, Ruppell's Warbler Sylvia ruppelli, Short-toed Lark Calandrella brachydactyla, Tawny Pipit Anthus campestris, Red-throated Pipit Anthus cervinus, Bluethroat Luscinia svecica, Black-eared Wheatear Oenanthe hispanica, Ortolan Bunting Emberiza hortensis, Collared Pratincole Glareola pratincola, Short-toed Snake-Eagle Circaetus gallicus, Egyptian Vulture Neophron percnopterus, Black-tailed Godwit Limosa limosa Barn Swallow Hirundo rustica (in West Africa), Tree Pipit Anthus trivialis, Woodchat Shrike Lanius senator, Common Nightingale Luscinia megarhynchos, Common Redstart Phoenicurus phoenicurus, Great Reed-Warbler Acrocephalus arundinaceus, Melodious Warbler Hippolais polyglotta, Wood Warbler Phylloscopus sibilatrix, Willow Warbler P. trochilus (in West Africa), Garden Warbler Sylvia borin Sand Martin Riparia riparia, Tree Pipit Anthus trivialis, Great Reed Warbler Acrocephalus arundinaceus, Sedge Warbler A. schoenobaenus, Willow Warbler Phylloscopus trochilus Garden Warbler Sylvia borin, Greater Whitethroat S. communis, Marsh Warbler Acrocephalus palustris, Thrush Nightingale Luscinia luscinia Spotted Flycatcher Muscicapa striata, Collared Flycatcher Ficedula albicollis, Barn Swallow Hirundo rustica, Hobby Falco subbuteo, Red-footed Falcon Falco vespertinus, Amus Falcon Falco amurensis, Common Cuckoo Cuculus canorus, Icterine Warbler Hippolais icterina, Eurasian Golden Oriole Oriolus oriolus

Note: During their pause in northeast Africa, many migrants moult, including Great Reed Warbler, Sedge Warbler, Olivaceous Warbler, Greater Whitethroat (eastern race, icterops) and Marsh Warbler (Pearson 1973, 1975, 1990, Pearson & Backhouse 1976, 1983, Pearson et al. 1988). Mainly from Moreau (1972) and Jones (1995, 1999).

In East Africa, forest is much more patchy than in the west, and the migrants encounter suitable woodland or savannah habitats from the Sahel southward into South Africa. Movements are longer than in the west, and a greater proportion of insectivorous species crosses the equator to winter in southern Africa (Lack 1990, Pearson 1990); their movements are not broken by any barrier equivalent to the Gulf of Guinea in the west.

Most of the birds that migrate to East and southern Africa derive from Asian breeding areas at 30-80°E, but some European birds move southeast to join their conspecifics from Asia in the Middle East, before passing down the eastern side of Africa. This pattern is evident in the Thrush Nightingale Luscinia luscinia, Marsh Warbler Acrocephalus palustris, Eurasian River Warbler Locustella fluviatilis, Olive-tree Warbler Hippolais olivetorum, Barred Warbler Sylvia nisoria, Red-backed Shrike Lanius collurio and Lesser Grey Shrike Lanius minor (Moreau 1972, Pearson 1990). In these species, populations from the whole breeding range spend the northern winter in eastern or southern Africa.

As in West Africa, many species spend the first few weeks after arrival in the Acacia savannahs between 12°N and 15°N (Ethiopia-Sudan) in areas where it has recently rained, and some species moult there (Table 24.3) (Pearson 1973, Jones, 1995). They then move on in November-December as the dry season sets in, some (mainly ground-feeders) to equatorial eastern Africa (Kenya-Tanzania) where the second wet season is just beginning (Lack 1983), and others (mainly vegetation-gleaners) cross the equator to southern Africa. Some of the latter undergo a second phase of pre-migratory fattening to take them to wintering grounds 2000-4000 km away (Pearson 1990, Pearson & Blackhurst 1976, Lack 1983, Jones 1985, 1999). They thus perform a two-stage migration, with a 2- to 3-month break between stages.

In some of the species involved, onward movements seem to be obligate and endogenously controlled. Such birds leave East Africa at about the same time each year, having accumulated migratory fat before departure. In other species, the onward movements may be facultative, as many more birds remain in northern Africa in wet than in dry years (Lack 1983), and are more variable in their timing of southward departure. Similarly, while some species (such as Red-backed Shrike Lanius collurio) are consistent in their arrival times in the Kalahari Basin of Botswana, other species (such as Great Reed Acrocephalus arundinaceus and Sedge Warbler A. schoenobaenus) show big year-to-year differences in arrival times related to variations in the onset of the local rains (Herremans 1994, 1998).

The centre of gravity of the collective migrant population thus shifts southward within Africa during the course of the northern winter, but over a shorter mean distance in the west than in the east. Some species take up to five months to move between their breeding areas and the farthest parts of their wintering range, travelling in two or more well-separated stages. In contrast, they take less than two months for their return journey, beginning in late March-early April, breaking their travels usually for only a few days at a time, and fattening rapidly (Pearson 1972, Ash 1980, Pearson & Lack 1992, Jones 1995). Throughout their journeys, most species seem to migrate by long flights, up to 1000 km or more, with fuel loads of 30-50% above their lean weights. Like the irruptive migrants of northern regions, some migrants to Africa extend further south in years when food is scarce, their numbers in particular localities fluctuating greatly from month to month and year to year, depending on patterns of rainfall, which affect food supplies (McLachlan & Liversidge 1978, Lack 1983, Liversidge 1989, Herremans 1998). In some years, they hardly appear at all in the southern parts of their wintering range, even though local food supplies may be good, presumably because in those years the whole population is accommodated further north (see Liversidge 1989 for Steppe Eagle Aquila nipalensis). Moreover, in some species the distribution of the population in southern Africa varies from year to year within the range, depending on where sporadic rainfall has created suitable conditions (Herremans 1998). Typically, such species move with the rain, seldom staying for long in any one locality, and in drought years not appearing at all (e.g. Lesser Spotted Eagle Aquila pomarina, Liversidge 1989). They are ultimately dependent on food supplies that result from fresh rain, whether grasshoppers and locusts, emerging termites or Quelea colonies. Although most such species are site-faithful in their Eurasian breeding areas, occupying the same territories throughout their stay year after year, they may be classed as irruptive or nomadic in their African non-breeding areas (Chapter 16).

In these various ways, some migratory species may be on the move for much of the time between leaving their breeding areas in one year and returning there the next, on a long circuitous journey, more or less repeated year after year. In the process, individuals of some species may spend successive periods in different localities, but may return to the same series of localities year after year. As shown by ringing, individuals of some passerine species may pass through the same spots, or return to the same clump of bushes, in successive years (Moreau 1972, Curry-Lindahl 1981). In addition to their nesting territories in Eurasia, individual birds thus remember and seek out two or more territory sites hundreds or thousands of kilometres apart in Africa, which they visit each year in orderly succession as each in turn offers suitable conditions through the changing seasons (Jones 1985). However, the time spent on each of these areas may vary greatly from year to year, depending on conditions at the time.

The strategy of using more than one area during the course of the winter (other than for mere stopovers) was called itinerancy by Moreau (1972). It is clearly a common form of behaviour in many migratory species in Africa, each leg of the journey being separated by up to several weeks from the next, and requiring a separate period of fat deposition.

Both southward and northward migrations through East Africa appear channelled by mountains, and there is marked east-west segregation of species, with some species taking one route and others a different route. Some species show a strong passage mainly through Uganda (e.g. Willow Warbler Phylloscopus tro-chilus, Garden Warbler Sylvia borin, Red-backed Shrike Lanius collurio) and others through eastern Kenya (Marsh Warbler Acrocephalus palustris, Greater Whitethroat Sylvia communis, Thrush Nightingale Luscinia luscinia). This segregation depends partly on breeding origin and partly on preferred habitat in Africa. Some species seem to pass through Kenya on a front only about 100-200 km wide, the Marsh Warbler Acrocephalus palustris being a striking example (Pearson & Lack 1992). On their return northward journey in spring, many birds take a more easterly, coastal route, associated with the more humid conditions there at that season, the whole migration forming a loop. Examples include Red-backed Shrike Lanius collurio and Lesser Grey Shrike Lanius minor, Great Reed Warbler Acrocephalus arundinaceus, Sedge Warbler A. schoenobaenus and Willow Warbler Phylloscopus trochilus, Garden Warbler Sylvia borin, Spotted Flycatcher Muscicapa striata and Thrush Nightingale Luscinia luscinia (Pearson 1990, Pearson & Lack 1992).

Whatever the particular migration strategies of different species, they more or less conform to a single pattern, evident throughout Africa: north of the equator, birds follow the rainbelt northward from March-April onwards, returning southward in October-November as the rains withdraw; south of the equator birds move further south with the rains from October-November, returning northwards in March-April. The pattern is essentially the same whether birds remain in the northern or southern tropics or whether they cross the equator. It is also the same whether the birds are Eurasian-African migrants or intra-African migrants (Elgood et al. 1973, Jones 1985). All are subject to the same ecological pressures. The most obvious difference between the two groups is that the Eurasian migrants move much further north than the African ones reaching the Palaearctic to breed. As the Eurasian migrants return to Africa in autumn, to some extent they replace the African migrants in the Sahel and Sudan zones which then move further south (Jones 1995, 1998). The extent to which the two groups replace each other ecologically awaits more study.

Was this article helpful?

0 0

Post a comment