Population limitation in Palaearctic‐African migrant passerines

We report the results of key factor analyses for Sedge Warbler Acrocephalus schoenobaenus, Blackcap Sylvia atricapilla, Whitethroat Sylvia communis and Willow Warbler Pftylloscopus Irochilus based on extensive data from the United Kingdom gathered by the British Trust for Ornithology. These results are compared with those from published key factor analyses based on intensive studies of Swallows Hirundo ruslica, Redstarts Phoenicurus phoenicums and three populations of Pied Flycatchers Fiiedula hypoleuca. Variation in losses between fledging and the following breeding season lk 4 ) accounted for most of the variation in total losses (K T ) for all populations. Breeding performance was poorly correlated with population changes. Density‐dependence was found mainly in 6 4 , its strength decreasing as average rate of population change increased. There was also evidence of weak density‐dependence in the clutch size of Swallows and in Willow Warbler egg losses. Fluctuations in k 4 were correlated with conditions on the wintering grounds in Sedge Warblers, Whitethroats and Swallows, for the first two species these relationships being confirmed by analyses of independent survival estimates based on mark‐recapture data. Populations of Sedge Warblers and Whitethroats appear to be limited by competition for resources on the wintering grounds. Removal experiments suggest that Willow Warblers, Pied Flycatchers and perhaps Blackcaps are limited through recruitment to the breeding population, in the case of Pied Flycatchers this limitation being in relation to the availability of nest sites.