Average migration speeds from populationbased ring recoveries

Average estimates are more useful than those obtained from individual flights, because they derive from many individuals and not just a few extreme ones. Ringing data indicate that Barn Swallows Hirundo rustica complete the 10 000 km journey between northern Europe and southern Africa at an average speed of at least 150 km per day (as against individual ringing recoveries which indicate exceptional speeds up to 340 km per day) (Alerstam & Lindström 1990). Other estimates relate to Spotted Flycatchers Muscicapa striata between southern Africa and Fennoscandia (spring) at 140 km per day (Fransson 1986), and to Willow Warblers Phylloscopus trochilus between Fennoscandia and Africa (autumn) at 85 km per day, both over distances exceeding 10 000 km (Hedenström & Pettersson 1987).

Birds do not necessarily maintain the same rate of progress over their whole migration. To judge from ring recoveries, some species speed up during the course of their journey, while others slow down. For Willow Warblers Phylloscopus trochilus travelling from northern Europe to Africa, mean speeds were estimated at 41 km per day over distances of 400-1000 km (N = 37), 54 km per day over distances of 1000-2000 km (N = 30), 59 km per day over distances of 2000-3000 km (N = 40) and 85 km per day over distances greater than 3000 km (N = 22) (Hedenström & Pettersson 1987). Similarly, for birds recovered in the first three weeks after ringing in autumn, mean migration speeds increased with distance in Bluethroats Luscinia svecica, Sedge Warblers Acrocephalus schoenobaenus, Reed Warblers A. scirpaceous and Lesser Whitethroats Sylvia curruca from Sweden, and in Garden Warblers S. borin and Blackcaps S. atricapilla from Britain (Ellegren 1990a, 1993, Fransson 1995, Bensch & Nielson 1999). Migration speed increased more rapidly through the journey in long-distance migrants wintering in Africa than in short-distance migrants wintering within Europe (Alerstam 2003). The reasons are unknown, but weather-induced delays may be fewer as birds reach lower latitudes.

On the other hand, other Sylvia warblers ringed in northern Europe and Britain seemed to slow down during their autumn passage through Europe. Speeds exceeding 100 km per day were recorded mainly in the first 10 days after ringing and, after about 20 days from ringing, average speeds had dropped markedly (Fransson 1995). Much depends on the nature of the migration, whether by long flights and long stopovers, or short flights and short stopovers. Whatever the migration mode, however, speeds calculated over the early parts of a journey (which often provide most ring recoveries) may not be typical of the whole route, for birds may already have accumulated some migratory fuel when they were caught and ringed. In addition, for obvious reasons, journeys over seas or deserts are almost always undertaken more rapidly than equivalent journeys through more favourable terrain.

In a process so dependent on food and weather, migration speeds would be expected to vary greatly from year to year over the same route. Annual variations have been little studied, but over the period 1963-1998 a four-fold variation in average migration speeds was recorded among Blue Tits Parus caeruleus travelling between various bird-ringing stations along the southern Baltic coast

Figure 8.4 The course of migration of European Marsh Warblers Acrocephalus palustris to South African wintering areas. The whole southward journey takes at least five months, and lines show progress, as revealed by ring recoveries (Dowsett-Lemaire & Dowsett 1987). The return journey is faster, taking about three months. Map reproduced by permission of NISC, publishers of Ostrich.

Figure 8.4 The course of migration of European Marsh Warblers Acrocephalus palustris to South African wintering areas. The whole southward journey takes at least five months, and lines show progress, as revealed by ring recoveries (Dowsett-Lemaire & Dowsett 1987). The return journey is faster, taking about three months. Map reproduced by permission of NISC, publishers of Ostrich.

(Nowakowski & Chrusciel 2004). In the slowest year, the birds averaged less than 10 km per day, compared with 38 km per day in the fastest year. In most years, the average speed was in the range 25-35 km per day, making the Blue Tit relatively slow among European passerines. This may be associated with its habit of migrating in short flights just above the tree-tops. The annual variations in speed paralleled those in the Great Tit P. major, which, however, migrated generally faster. Interestingly, Coal Tits P. ater studied in the same way migrated twice as fast in invasion years (40-80 km per day) as in other years (30-40 km per day), possibly because the migratory drive was stronger in invasion years (Rute 1976), a difference also noted in Siskins Carduelis spinus (Payevski 1971).

Most of the estimates of migration speed obtained from ring recoveries refer to autumn, when juveniles predominate, and the few records from spring suggest that faster progress is made then, at least by passerines and shorebirds. Thus, Barn Swallows Hirundo rustica from Britain take an average of 10 weeks to reach South Africa in autumn, travelling at 150 km per day, and 5-6 weeks to return in spring, travelling at 300 km per day (Mead 1970). Similarly, Marsh Warblers Acrocephalus paludicola migrating between Europe and South Africa were estimated to take 20 weeks over the autumn journey, and 12 weeks over the spring journey (Figure 8.4).

Table 8.1 Speed of migration (km per day) of five Sylvia species from northern Europe and Great Britain, estimated according to differences in median trapping dates and median dates of recoveries in the Mediterranean area

Species Great Britain Northern Europe

Autumn Spring Autumn Spring

Table 8.1 Speed of migration (km per day) of five Sylvia species from northern Europe and Great Britain, estimated according to differences in median trapping dates and median dates of recoveries in the Mediterranean area

Barred Warbler Sylvia nisoria

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