Differences in Age and Sex Ratio among Migrating and Wintering Raptors in Southern Sweden

Abstract

-In order to test hypotheses explaining intraspecific differences in migration patterns of birds, a four-year field study of migrant raptors in southernmost Sweden was undertaken. The age and sex ratios among autumn migrants departing from the province of Scania at Falsterbo were compared to the corresponding ratios among birds of the same species that stayed in Scania the following winter. In most species (Red Kite [Milvus milvus], White-tailed Eagle [Haliaeetus albicilla], Northern Harrier [Circus cyaneus], Goshawk [Accipiter gentilis], Sparrowhawk [A. nisus], Common Buzzard [Buteo buteo] and Eurasian Kestrel [Falco tinnunculus]), the proportion of adults was distinctly higher among wintering birds than among the migrants. A marked exception was the Rough-legged Hawk (B. lagopus), which had significantly more juveniles among the wintering birds. In most species where the sexes could be separated more females were found among the wintering birds than among the migrants (Northern Harrier, Goshawk, Sparrowhawk and Rough-legged Hawk). The Eurasian Kestrel, however, showed the reverse pattern with a higher proportion of males among wintering birds. The results support the social-dominance hypothesis, where the dominant adults and the larger sex (in raptors the female) generally winter furthest north. That juveniles are driven away from the breeding territories is suggested by a significantly higher proportion of juvenile Common Buzzards wintering in marginal coastal regions as compared to the inland portion of Scania. The results are also in general agreement with Bergmann's rule. The higher proportion of male Eurasian Kestrels among wintering birds is most likely explained by the importance of early arrival at the breeding territory. Received 6 July 1992, accepted 25 November 1992. AMONG BIRDS we find a mixture of migration strategies from completely resident species to long-distance migrants travelling thousands of kilometers between summer and winter quarters each year. Migration can be expected to occur in populations where birds survive in greater numbers if they leave their breeding areas for the nonbreeding season than if they remain there for the whole year (Lack 1954). Even within species there may be great variation. In the Common Buzzard (Buteo buteo), birds breeding in northeast Europe are long-distance migrants spending the winter in southern Africa, while birds breeding in most parts of Scandinavia are short-distance migrants wintering on the European mainland, and breeding birds in southern and western Europe are residents (Cramp et al. 1980). Within species, various categories of birds (e.g. age and sex classes) clearly have different migration patterns. This so-called differential migration has attracted the attention of several authors (Gauthreaux 1982, Ketterson and Nolan 1983, Lundberg 1988). Several hypotheses have been put forward to explain interand especially intraspecific differences in migratory and wintering patterns in birds. Compilations have been made by Gudmundsson (1988) and Kerlinger (1989) and a short summary follows: (1) The body-size hypothesis, closely related to Bergmann's rule (Salomonsen 1955), states that large birds are able to winter further north since they are better able to tolerate cold surroundings (Searcy 1980). As a result, smaller individuals can be expected to undertake longer migrations than larger ones. (2) The social-dominance hypothesis (Cox 1968, Mueller et al. 1977, Gauthreaux 1978, 1982) proposes that, in short-distance migrants, subordinate individuals are forced to migrate from the breeding grounds (and/or the best wintering areas) because of competition from more dominant birds. (3) The arrival-time hypothesis (King et al. 1965, Myers 1981) indicates that, if there is competition for breeding resources, birds returning earlier to the breeding grounds do better. Therefore, it is advantageous to winter as close to the breeding area as possible. (4) The character-divergence hypothesis (Koplin 1973) explains differences in winter habitat as being a result of character displacement due to