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

Nils Kjellén
The Auk
Vol. 111, No. 2 (Apr., 1994), pp. 274-284
DOI: 10.2307/4088592
Stable URL: http://www.jstor.org/stable/4088592
Page Count: 11
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Differences in Age and Sex Ratio among Migrating and Wintering Raptors in Southern Sweden
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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.

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