Different strategies for similar journeys

Closely related species migrating between the same areas sometimes show different patterns of fuelling and flight, depending partly on their food supplies. For example, Sedge Warblers Acrocephalus schoenobaenus travelling from Britain to Africa in autumn attain higher rates of fat deposition close to their breeding areas than further south in the Mediterranean region. Their main prey (plum reed aphids) reach peak abundance in Britain and northern France at migration time, but they have already passed their peak in southern Europe by the time the warblers arrive. This may be why Sedge Warblers typically deposit very large fat loads in southern England and northern France, from which areas many may then make a single long flight to their wintering areas south of the Sahara, a journey exceeding 3000 km (Gladwin 1963, Bibby & Green 1981). In contrast, Reed Warblers Acrocephalus scirpaceus eat a wider variety of insects, and can fatten at a wider range of sites until later in the season. Ring recoveries show that they usually migrate through Europe in shorter stages, stopping at various localities on their southward journeys, and accumulating large body reserves only in North Africa, just before the desert crossing. Consequently, while both these warblers make the same journey, the location of refuelling stopovers, and patterns of fattening and flight lengths differ between them. For some bird species, including the Sedge Warbler, much otherwise suitable habitat may contain insufficient food at migration time to allow efficient fuelling, leading to a high fuelling/long flight strategy even over apparently favourable terrain.

Analyses of the weights of six species of trans-Saharan migrants caught at 34 trapping stations located in widely scattered parts of Europe and North Africa revealed four types of fattening patterns on the southward autumn journey (Schaub & Jenni 2000):

1. Birds accumulate large fuel stores well before they reach the northern edge of the desert, and then fly to sub-Saharan Africa without refuelling. This pattern is shown by western populations of Sedge Warblers Acrocephalus schoenobaenus (as just mentioned), by western populations of Pied Flycatchers Ficedula hypo-leuca, and possibly also by some individuals of other species.

2. Birds accumulate more fuel at each stopover than is needed to fly to the next, leading to a progressive increase in body mass southwards through Europe toward the desert. This strategy is shown by Garden Warblers Sylvia borin and more eastern populations of Pied Flycatchers Ficedula hypoleuca.

3. Birds migrate in short stages, and accumulate only enough fuel at each stopover site to fly to the next, with especially large amounts just before the desert crossing. This strategy depends on finding good feeding sites in the southern Mediterranean region, and is adopted by Eurasian Reed Warblers Acrocephalus scirpaceus (as mentioned above) and possibly also by Greater Whitethroats Sylvia communis.

4. The same strategy as (3), except that birds put on only moderate reserves before the desert crossing, relying instead on finding food at desert oases or on catching migrant insects. Various hirundines and Spotted Flycatchers Muscicapa striata seem to adopt this strategy, and the same may be true for shrikes which can also kill and eat their fellow migrants. However, Barn Swallows Hirunda rustica that migrate from Italy across the Mediterranean and Sahara accumulate up to 40% fat before the journey, which is much more than those that take the shorter sea crossing at Gibraltar (Rubolini et al. 2002).

Similar patterns occur in spring (Curry-Lindahl 1963, Ward 1963, Fry et al. 1972, Wood 1992). Thus, while most northward-bound passerine populations fatten well to the south of the Sahara, and undertake a flight much longer than the desert crossing itself, others proceed in stages to the southern edge of the desert and fatten there. In East Africa, Sedge Warblers Acrocephalus schoenobaenus and Great Reed Warblers Acrocephalus arundinaceus fatten well south of the Sahara (in Kenya-Uganda) and fly 2500 km direct to the Middle East; but most other passerine species probably fatten in Somalia much nearer to the Sahara (Pearson 1990). In West Africa, Garden Warblers are among those that fatten far south of the desert, in the Guinea Zone (Ottosson et al. 2005), while Sand Martins Riparia riparia and others fatten around Lake Chad in the Sahel Zone (Fry et al. 1972). The migratory stage that involves crossing the Sahara is relatively rapid (less than one week), and later stages through Europe are slower (three or more weeks).

In North America, Helms & Smythe (1969) recognised similar broad categories with respect to fuel reserves and migration itineraries:

1. Intra-continental migrants vary from (a) those that depart on autumn migration with scant fuel reserves, move relatively slowly, and may (e.g. Dark-eyed Junco Junco hyemalis, Savannah Sparrow Passercualus sandwichensis) or may not (e.g. Tree Sparrow Spizella arborea) add reserves as migration progresses, to (b) those that accrue moderate reserves immediately before departure and migrate fairly rapidly (e.g. White-throated Sparrow Zonotrichia albicollis in some areas).

2. Inter-continental migrants that behave like intra-continental migrants on the first part of their journey through favourable habitat, but accrue much larger reserves in the southern States as they approach a sea crossing (e.g. Scarlet Tanager Piranga olivacea and Bobolink Dolichonyx oryzivorus, Table 5.3).

In both types, breeding populations taking different routes may have different strategies, presumably developed according to conditions en route, including the spatial and temporal distributions of their habitats and food supplies. Again, the same populations may also adopt different strategies at the two seasons, and adults may differ from juveniles, but it is not known whether individuals can switch easily from one strategy to another as circumstances dictate.

At particular sites, birds sometimes stayed longer, fattened more rapidly and to higher levels in autumn than in spring (Dolnik & Blyumental 1967, Morris et al. 1994), and in other species in spring than in autumn (King et al. 1963, 1965, Butler et al. 1987). For example, some waders migrating between eastern Canada and South America accumulate more body fat in autumn, when they make a single long flight over the sea to northern South America, than in spring when they migrate by a series of shorter coastal flights, broken by feeding stops (McNeil & Cadieux 1972). Similarly, birds crossing the Sahara and fattening close to its edges tend to stay there longer and accumulate more body fuel in autumn, before the crossing, than in spring, just after it (Table 5.4). The fattening strategy at each season is evidently adapted to the route taken at each season and the numbers and spacing of potential fuelling sites.

Table 5.4 Mean estimated stopover periods in days of various passerines at an oasis in the Sinai desert. The average stopover period in most species was longer in autumn before crossing the Sahara than in spring after crossing it

Autumn Spring

Species present only in autumn

Willow Warbler Phylloscopus trochilus Orphean Warbler Sylvia hortensis Red-backed Shrike Lanius collurio Greater Whitethroat Sylvia communis Yellow Wagtail Motacilla flava Bluethroat Luscinia svecica Eurasian Reed Warbler Acrocephalus scirpaceus Whinchat Saxicola rubetra European Pied Flycatcher Ficedula hypoleuca

Species more numerous in autumn than spring

Lesser Whitethroat Sylvia currucaa Eurasian Chiff-chaff Phylloscopus collybita Spotted Flycatcher Muscicapa striata Tree Pipit Anthus trivialisa

Species more numerous in spring than autumn

Blackcap Sylvia atricapilla Redstart Phoenicurus phoenicurus Garden Warbler Sylvia borin aDifference between autumn and spring, P<0.001. From Lavée et al. (1991).

This site was so small (a small garden (0.02 km2) at St Catherine's Monastery surrounded by barren desert) that individual birds could be easily caught on arrival, and the length of stay of a sizeable proportion of birds could be estimated reliably to within narrow limits. It was situated 300 km from the desert's northern edge and about 2000 km north of the southern edge of the Sahara. In general, the longest stopovers were shown by birds on long migrations for which the stopover site was far from the point of initiation, while the shortest were in species for which the stopover site was close to the point of initiation in autumn.

Figure 5.2 Recoveries of birds ringed in northern Europe that were found in the eastern Mediterranean region. Lines connect the ringing site with the recovery site for each individual. Left: Thrush Nightingale Luscinia luscinia. Right: Blackcap Sylvia atricapilla. These are two out of seven passerine species which migrate from northern Europe round the eastern Mediterranean in autumn for which there are sufficient ring recoveries for analysis. From Fransson et al. (2005).

Figure 5.2 Recoveries of birds ringed in northern Europe that were found in the eastern Mediterranean region. Lines connect the ringing site with the recovery site for each individual. Left: Thrush Nightingale Luscinia luscinia. Right: Blackcap Sylvia atricapilla. These are two out of seven passerine species which migrate from northern Europe round the eastern Mediterranean in autumn for which there are sufficient ring recoveries for analysis. From Fransson et al. (2005).

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