From Fransson (1995).
On the basis of median recovery and trapping dates, four species of Sylvia warblers also showed much faster migration speeds in spring than in autumn for journeys between the Mediterranean region and the north European breeding areas (Table 8.1, Fransson 1995), as did Spotted Flycatchers Muscicapa striata through the same region (Fransson 1986). In contrast, Pied Flycatchers Ficedula hypoleuca travelled between northern Europe and tropical Africa at average speeds of 120-170 km per day in autumn but only at 100 km per day in spring (Lundberg & Alatalo 1992).
For many diurnal birds, fuelling rates may be influenced not only by the food supplies available en route, but also by the prevailing daylengths which determine the maximum daily feeding times, and hence the maximum possible migration speed (Kvist & Lindstrom 2000). In many bird species, spring migration occurs closer than autumn migration to the summer solstice, and hence over longer days (Chapter 14; Bauchinger & Klaassen 2005). The migratory speeds of three species of Sylvia warblers were, on average, 47% faster in spring than in autumn, calculated from ring recoveries of adult birds migrating between the Mediterranean and northern Europe. For these species, the spring journey was 34% faster in S. communis, 47% faster in S. borin and 59% faster in S. atrica-pilla (Fransson 1995). In parallel, the amount of daylight over the same migratory distance was 26% longer in spring than in autumn, calculated by comparing daylength at the mid point (52°N) of the migratory journey, and of the spring and autumn migration periods (15 May and 18 September respectively). A fourth species, S. curruca, showed no significant difference in migration speed between spring and autumn. If daylength has an influence (through its effect on fuelling), migration speed should change during the course of a journey, as the bird moves through latitudes with different daylengths. To some extent, the same would be expected in soaring birds which are limited each day by the period when ther-mals occur (both daylength and temperature dependent). Birds that migrate between hemispheres change daylength regime part way through the journey. Nevertheless, there will still be some specific departure date that is optimal for maximising the overall daylength to which the bird is exposed, and hence for minimising the duration of migration (Alerstam 2003).
Although we can estimate or measure the maximum migration speeds of birds, we should not assume that all birds are under pressure to migrate as fast as they are able, at least in autumn. Comparisons between different populations of some species indicate that other factors are involved, and that the time available for migration may also have an influence. For example, Common Kestrels Falco tinnunculus from northern Sweden travel twice as far as those from the south, but in the same time (Wallin et al. 1987). This difference held even though the entire journey of the short-distance birds lay within the route of the long-distance ones. In spring, the two populations migrated at the same speed, so the northern birds took longer to reach their breeding areas. Similarly, White Storks Ciconia ciconia from central Europe using the southwest route through Iberia take about three months to reach their winter quarters in West Africa. Those taking the southeast route through the Middle East to East Africa also take about three months to cover twice the distance (Figure 8.5, Bairlein 2001).
Analysing ringing data for four species of Sylvia warblers, Fransson (1995) discovered that British birds travel through Europe at slower average speeds (43-62 km per day) than Scandinavian birds (66-93 km per day). He attributed this to the fact that British birds have shorter total migration distance (and hence may be under less pressure) and a slightly different migration schedule from Scandinavian ones. These various findings suggest that some populations are not under pressure to migrate in autumn at the maximum speed of which they are capable. Perhaps at this time of year they have a certain period in which to migrate, according mainly to other events in the annual cycle, and can adjust their travelling speed accordingly. The benefits of migrating slowly are that birds can take advantage of rich feeding areas they encounter en route, yet do not need to accumulate the massive fuel reserves required by long flights, thereby avoiding the associated predation risks (Chapter 5).
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