Figure 8.3 Migration speeds recorded for passerines ringed in Finland in relation to the seasonal timing of migration as recorded at Ottenby in Sweden. The data clearly indicate the reduction of migration speed with advance in mean departure date, and the differences between different categories of migrants. From Alerstam & Lindström (1990), based on data in Hilden & Saurola (1982).
recovered up to 50 days later at various points on their migration routes (Hilden & Saurola 1982, Ellegren 1993, further analysed by Alerstam & Lindström 1990, Alerstam 2003). From these and other records (Figure 8.3), the following generalisations emerged:
1. For birds studied over at least 10 days, average migration speeds in the range 21-263 km per day were found for different species. In general, shorebirds migrated more rapidly (median 79 km/day, N = 13 species) than passerines (medians 27-75 km per day in different groups, see below) (Alerstam & Lindström 1990). This difference may be because shorebirds fly faster than passerines, are more selective of favourable winds, and migrate at higher altitudes where winds are stronger. In moving between successive staging areas, shorebirds typically fly 500-1000 km in 1-2 days, interrupted by several-day stops, whereas passerines typically migrate over land almost every day at a more even speed, with shorter flights and shorter stops. Maximum speeds up to 200-300 km per day were recorded in passerines, and up to 400-1000 km per day in shorebirds. These maximum speeds were consistent with theoretical expectations.
2. Among passerines, average migration speeds varied with autumn starting date, early-departing species travelling more rapidly than late-departing ones (Figure 8.3). Thus, speeds were considerably higher for long-distance species with tropical winter quarters that left early (median speed 75 km per day, n = 13) than for short-distance temperate zone migrants that left later (median speed 53 km per day, n = 19), or for irruptive and partial migrants that left latest of all (median 27 km per day, n = 11). This trend could result from seasonal declines in food supplies or daylengths, leading to reduced rates of fuel deposition from late summer into autumn (Alerstam & Lindström 1990, Ellegren 1993, Kvist & Lindström 2000). In addition, fast travel may be more important for early, long-distance species bound for the tropics than for later, short-distance species migrating only within Europe (Alerstam & Lindström 1990). Not only do the earlier migrants have further to go, but they probably gain from crossing the Sahara in August-September, before the dry season takes hold in the Sahel zone to the south.
3. In association with the above relationships, migration speeds were correlated with the length of journey (Figure 8.3). Juvenile passerines travelling 1000 km on migration covered about 80 km per day, on average, whereas those travelling 5000-6000 km covered about 140 km per day (Alerstam 2003). Longdistance migrants seem to migrate in longer steps, and are more selective of good weather conditions. In addition, long-distance migrants are generally nocturnal flyers, using time for flying that they would otherwise spend at roost, without reducing the time available for foraging (this would not apply to shorebirds which can feed day and night given suitable tidal conditions).
4. Among passerines, nocturnal migrants made faster progress, on average, than diurnal migrants (72 km per day vs. 53 km per day; samples sizes 17 and 6 species). In addition, migration flight lengths were longer in nocturnal (177 km) than in diurnal migrants (111 km), even though nocturnal migrants did not always use the entire night for flying (Cochran et al. 1967, Ellegren 1993).
5. Despite their faster migration speeds, Eurasian-Afrotropical migrants, with mean migration distances of 6000-10 000 km, still travelled for much longer autumn periods (median 88 days, n = 13) than regular temperate zone migrants (median 42 days, n = 10, distances 1700-3000 km), or than partial migrants (median 32 days, n = 11, distances 200-300 km).
6. Some extremely rapid migrations were recorded in some long-distance passerine migrants. For example, a Willow Warbler Phylloscopus trochilus travelled 8000 km from Finland to Congo within 56 days, a mean speed of 145 km per day (Hilden & Saurola 1982), while another Willow Warbler travelled from Finland to South Africa in 47 days, a mean speed of 218 km per day (Hedenstrom & Pettersson 1987). A Marsh Warbler Acrocephalus paludicola travelled 1400 km within Africa in five days, an average of 280 km per day (Cramp 1992); and three Barn Swallows Hirundo rustica travelled average distances of 250, 350 and 433 km per day over journeys of 8500, 12 000 and 3000 km respectively (Turner 2006). However, all these records refer to exceptional individuals, the majority of their species progressing much more slowly.
7. The decline in migration speed with advance in departure date did not apply within species. On the contrary, late migrants tended to progress more rapidly than early ones of the same species, at least in the initial part of the autumn journey (for various passerines see Ellegren 1993; for juvenile Reed Warbler Acrocephalus scirpaceous and Sedge Warbler A. schoenobaenus see Bensch & Nielsen 1999). This trend would give birds that were delayed in their departure from breeding areas an opportunity to 'catch up' during migration (Fransson 1995). Blue Tits Parus caeruleus migrating in autumn along the southern Baltic coast travelled an average of about 28 km per day during 15-24 September, increasing significantly to about 38 km per day during 15-24 October (Nowakowski & Chrusciel 2004).
8. In some species, adults travelled faster than juveniles. For example, adult Dunlins Calidris alpina took an average of 4.5 days to travel 660 km, while juveniles took 13 days over the same journey (Hilden & Saurola 1982). A similar age-related difference held in various Sylvia warblers migrating in autumn from northern Europe (Fransson 1995). By the time they start migrating, juvenile birds are to all appearances 'full-grown', and in this respect are almost as well equipped for migration as older birds.
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