Differential migration in raptors
Abstract: Differences between ages, sexes and populations in the timing of migration at Falsterbo, Sweden and in the choice of wintering site among raptors in Scania were investigated. Competition and dominance is the best primary explanation to the results. In most cases the more competitive adults winter north of juveniles and the larger females north of males. Moult strategy is probably one of the most important reasons why females in general migrate south earlier than males, and juveniles ahead of adults, in some short-distance migrants. The proportion of adults and juveniles in Swedish raptors leaving via Falsterbo is calculated. The concentration rate is generally highest in species with a more southerly distribution and in raptors primarily using thermal migration. With a few exceptions juveniles are more concentrated at Falsterbo than adults. An analysis of the long-term systematic migration counts from Falsterbo, shows that in all raptors where the Swedish population trends are reasonably well known from other studies, the fluctuation in numbers is very well reflected in the autumn figures from Falsterbo. The proportion of juvenile birds among the migrants gives a good measure of the annual reproduction in several species. By separating Common Buzzards with a white plumage (mainly with a relatively southerly breeding range) at Falsterbo it was possible to demonstrate some differences in migration pattern compared to darker and more northerly birds. Studies of three species of skuas on the Russian tundra demonstrated interesting differences in the proportion of dark phase. While no dark phase birds seem to occur in Long-tailed Skua the proportion was close to 8 % dark birds in Pomarine Skua all the way from Norway to Wrangel Island. This strongly indicates a gene flow all along the transect in this nomadic species. In Arctic Skua on the other hand there was an abrupt change from 64 % dark phase on the Kola Peninsula to an almost complete dominance of light birds all along the Russian tundra. Satellite transmitters constitute an invaluable tool enabling detailed studies of the migration strategies of individual birds. Thirteen Ospreys were tracked on autumn migration from Sweden to the wintering grounds in tropical Africa. The average speed for the whole journey was 174 km/day. This is similar to migration speeds in other raptors studied by satellite telemetry, but considerably faster than results from ringing recoveries. There was large individual variation with few major differences between sexes and ages. The birds made between 0 and 4 stopovers along the way. Migration took place only during the light hours and the Ospreys spent about nine hours in active flight on travelling days.
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