Population limitation in Palaearctic‐African migrant passerines

Abstract

We report the results of key factor analyses for Sedge WarblerAcrocephalus schoenobaenus,BlackcapSylvia atricapilla,WhitethroatSylvia communisand Willow WarblerPftylloscopus Irochilusbased 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 SwallowsHirundo ruslica,RedstartsPhoenicurus phoenicumsand three populations of Pied FlycatchersFiiedula hypoleuca.Variation in losses between fledging and the following breeding seasonlk4) accounted for most of the variation in total losses (KT) for all populations. Breeding performance was poorly correlated with population changes. Density‐dependence was found mainly in 64, 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 ink4were 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.