Autumn migration and weights of BlackcapsSylvia atricapillaand Garden Warblers S.borinat an inland site in southern England

Abstract

At a site in Buckinghamshire, England, in 1990–1992, numbers of Blackcaps peaked in late September, while Garden Warbler numbers declined steadily through the autumn. Both species showed an increase in mean weight through the autumn, due to the premigratory deposition of fat. About 30% of Blackcaps, but only 5% of Garden Warblers, weighed over 20g (ca. 25% fat). The results indicate that the heaviest 25% of Garden Warblers could have flown as far as north Portugal without refuelling, and the fattest 25% of Blackcaps as far as central‐southern Portugal. These results are discussed in relation to the two species’ migration strategies and patterns of occurrence at British bird observatories.