Autumn (km per day)
Figure 8.10 Relationship between the durations and speeds of the autumn and spring migrations of different raptor populations, based on the mean values from different studies listed in Table 8.3 (but excluding the White-tailed Eagle which took markedly different routes in autumn and spring). The line on each graph shows the relationship expected if the two migrations were of equal duration and speed. Overall, no significant difference was apparent in the duration or speed between seasons (on a paired t-test for duration, t = 0.08, P = 0.94; for speed, t = -0.93, P = 0.37).
(81 km per day). However, even including this bird, the regression line relating the duration of autumn and spring journeys was not significantly different from one, which implies equality in the mean duration of both journeys.
A striking feature in those species with reasonable samples was again the large individual variation in the duration of migration. Among Ospreys Pandion haliaetus that breed in Sweden and winter in West Africa, the total length of journey varied between 5813 and 7268 km (longest journey 1.25 times longer than the shortest), but this journey took between 14 and 55 days to complete (greatest value 3.9 times longer than shortest) in a sample of 13 individuals (Kjellen et al. 2001). Mean speeds varied between 108 and 431 km per day. Among Honey Buzzards Pernis apivorus over the same route, the journey varied between 6299 and 7091 km (longest journey 1.13 times longer than the shortest), and took between 34 and 70 days to complete (greatest value 2.06 times longer than the shortest) in a sample of nine individuals (Hake et al. 2003). Mean speeds varied between 93 and 209 km per day. In other species, equivalent journeys took up to three times longer in some individuals than others (Table 8.3).
Over the same journey, no difference was apparent in the autumn migration speeds of juvenile and adult Ospreys. In Honey Buzzards the average speeds of adults and juveniles on travelling days were similar, about 170 km per day in Europe, 270 km per day across the Sahara and 125 km per day in Africa south of the Sahara. However, as the adults had fewer stopover days en route, they maintained higher overall speeds and completed migration in a shorter time (42 days) than the juveniles (64 days) (Hake et al. 2003). The spring journeys of several eagle species were slower in juveniles than in adults, mainly for the same reason (Table 8.3). Although Peregrines and Ospreys often progress by flapping flight when soaring is not possible, their mean migration speeds were within the range of values recorded for other soaring species in Table 8.3. In general, within regional populations, those adults that spent the longest periods on migration spent the shortest periods in their wintering areas, while no effect was apparent on the amount of time spent in breeding areas.
Turning to non-raptorial species, White Storks Ciconia ciconia covered the first part of their autumn migration from Europe to East Africa by almost daily travelling. They took an average of 18.9 days over this part of their journey, which included only 1.4 non-travelling days (93% of all days were travelling days), giving an overall speed of 243 km per day (Berthold et al. 2001b). No differences were apparent between adults and juveniles. The storks did not eat excessively before their departure, but seemed to forage opportunistically in the mornings and evenings en route, regaining any net loss in body mass at their intermediate destination in northern Africa (mostly Sudan). Some individual White Storks showed great variation in the timing, length and duration of their migratory journeys from year to year, as well as in the duration and location of stopovers. One adult female was followed on six outward and six return journeys from its breeding place in Germany to its wintering place in Africa, which in two years was within a few degrees of the equator and in four years in South Africa. This bird took 121-169 days over the southward journey (mean 149 days) and 45-115 days over the return journey (mean 69 days) (Table 8.4). Its starting dates from the nesting area in different years varied by up to 15 days, and from the wintering area by up to 25 days. The longest stopover (in autumn) lasted 75 days, apparently in an area with abundant food (Berthold et al. 2004). Mean speeds in different autumns varied from 56 to 92 km per day (average 74 km per day) and in different springs from 97 to 165 km per day (average 147 km per day). The autumn journeys thus took longer, but chiefly because of the extended breaks in northern Africa which were not repeated in spring. Another population of White Storks in eastern Asia migrated through mainly favourable habitat (green river valleys), and radio-tagged individuals travelled much more slowly than the European-African birds, averaging only 28 km per day, and less than 10 km on most days, providing a clear demonstration of the effect of terrain on migratory behaviour (Higuchi et al. 2000).
Among various species of cranes, mean speeds in the range 33-146 km per day were recorded, somewhat slower than the raptors and storks, but cranes often migrate by flapping flight and stop for longer periods to refuel (Table 8.3). Demoiselle Cranes Grus virgo provided an exception, however, as the individuals tracked migrated over mainly high desert and the Himalayas, with few potential feeding places, and completed the main part of their journey within a week, averaging 350 km per day (Kanai et al. 2000). This flight may have been preceded by a long fuelling period.
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