Migration Mode

For many birds it may be advantageous to migrate as rapidly as possible, and thereby minimise the time spent on the journey (the 'time minimisation model' of Alerstam & Lindstrom 1990). This gives migrants the longest possible time on their breeding, wintering or moulting sites, but requires large fuel stores to permit long, non-stop flights. Other birds may have food available throughout the migration route, so that they can stop and feed almost anywhere. Because heavy fat loads mean greater transport costs, as mentioned above, one way to save energy is to keep fat loads small and fly only short distances at a time, refuelling as necessary (the 'energy minimisation' model). Moreover, because any extra weight also reduces flight performance (notably climb rate and agility), minimising fuel loads can also reduce predation risks (the 'predation minimisation' model). The second and third options may thus be combined as the 'load-minimising' strategy. The particular migration mode adopted by any population might be a compromise between any of these different options, depending partly on the type of terrain over which populations travel, the distribution of potential feeding places, and the risks of predation. Moreover, any 'ideal strategy' that the bird might have is likely often to be compromised by external conditions, such as adverse weather and poor food supplies.

Where birds follow a 'stepping stone' migration strategy, this may occur in different forms (called hop, skip and jump by Piersma 1987), depending mainly on the distances between successive feeding sites. For a load minimiser, the best strategy would be to use a large proportion of potential refuelling sites along a migratory route (hopping). But a time minimiser would do better to put on a large fuel load at a high-quality stopover site, in order to bypass a poor-quality stopover site (skipping). It could then migrate more quickly (Gudmundsson et al. 1991). For example, Bewick Swans Cygnus columbianus stop at the White Sea in spring but mostly bypass this site in autumn when their fuel reserves enable them to travel to a more distant site along the migration route (Beekman et al. 2002). A bird that has to cross a large stretch of inhospitable terrain can only adopt a 'jump' strategy, flying a long distance without feeding. Studies of the duration of stopovers, rates of weight gain and departure weights of migrants making overland journeys give some idea of the strategy pursued, bearing in mind that other factors also influence the behaviour of migrants.

Seabirds migrating entirely over the sea would seem to have plenty of opportunity to pick up food en route. But this is not always the case. Many species breeding at high latitudes migrate over the equator, and tropical seas are notoriously poor in food. In any case, foods such as fish tend to be concentrated in particular localities, which may be few and far between. Evidence is accumulating that, like some landbirds, some seabirds refuel at traditional staging areas before continuing migration. For example, after breeding in western Europe, Black Terns Chlidonias niger assemble at one major feeding area, the IJsselmeer on the Dutch coast. Here they increase in body mass by 25-30% within 2-3 weeks, which would then enable a non-stop flight of more than 3600 km to West Africa. The birds ascend in the evening to high altitudes (>500 m) and start migrating at night. Although Black Terns are seen at localities en route, no important stopover site is known between the IJsselmeer and West Africa (van der Winden 2002). In Namibia, a similar increase in body mass of these terns was noticed just before spring migration. Other terns may also make long flights between regular rich feeding areas, rather than hunting as they travel. Arctic Terns Sterna paradisaea staging in Norway and Britain in autumn would need to accumulate fuel equivalent to 30-40% of body weight before conducting a direct flight of 3000-5000 km to West Africa (Alerstam 1985). These and other seabirds (mentioned later) make their journeys so quickly that they can spend little (if any) time feeding en route.

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