DIFFERENTIAL TIMING OF AUTUMN MIGRATION BETWEEN SEX AND AGE-GROUPS IN RAPTORS AT FALSTERBO, SWEDEN

The migration of raptors at Falsterbo, Sweden was studied during five autumns, 1986-1990, to investigate the intraspecific temporal sequence of migration for different age and sex classes. The analysis is based on counts of between 45 and 72 321 individuals of 14 species. The percentage of juveniles varied among species and to some extent between years. This reflects a variable tendency of different migrants to concentrate at Falsterbo and fluctuating breeding success from year to year. Most species showed an even sex ratio. Marked exceptions were Marsh Harrier Circus aeruginosus and Hen Harrier C. cyaneus in which females constituted 60 and 61% respectively of adult migrants. This biassed sex ratio agrees with the tendency towards polygyny in these species. In tropical (long-distance) migrants adults migrated ahead of juveniles (Honey Buzzard Pernis apivorus, Montagu's Harrier Circus pygargus, Osprey Pandion haliaetus, and Hobby Falco subbuteo). It is suggested that it is advantageous for adults to arrive early on the wintering grounds, finish moult and accumulate energy reserves before spring migration and the next breeding season. Those short-distance migrants in which adults migrated before juveniles at Falsterbo are species whose majority leave Scandinavia in the winter (Hen Harrier, Common Buzzard Buteo buteo, Rough-legged Buzzard B. lagopus, Merlin Falco columbarius and Peregrine F. peregrinus). Adults leaving first have a greater possibility to secure a good winter territory forcing the juveniles, migrating later, to settle in less suitable habitat or to winter further south. In species where juveniles migrated ahead of adults a large proportion of the adults spend the winter close to the breeding grounds (Red Kite Milvus milvus, Goshawk Accipiter gentilis and Sparrowhawk A. nisus). The presumably subdominant juveniles may be excluded from the breeding territories by the adults and therefore migrate south, followed by a varying proportion of adults. Females generally migrated before males at Falsterbo due to different moult strategies; females starting and finishing moult of the flight feathers before males. This difference in timing of moult and migration is ultimately dependent on a marked role division between the sexes during the breeding cycle. In Honey Buzzard where both sexes share the breeding duties equally no difference in timing of migration was found. Social dominance, importance of early arrival at the winter quarters and moult strategies are the factors best explaining the observed differential timing in migrating raptors at Falsterbo.