Is phenotypic variation of blue tits (Parus caeruleus L.) in Mediterranean mainland and insular landscapes adaptive?

Abstract

Few empirical studies of bird populations have examined in detail how proximate and ultimate factors determine phenotypic variation in relation to small-scale environmental variation. This question has been addressed in the Mediterranean region, using blue tit (Parus caeruleus) populations living in habitat patches dominated either by deciduous (Quercus humilis) or evergreen (Q. ilex) oaks, which strongly differ in the timing and abundance of food resources. Our aim was threefold: 1) to analyse phenotypic variation and local differentiation of fitness-related life history traits on the scale of habitat mosaics within two different landscapes, one in mainland southern Prance and one on the island of Corsica; 2) to examine to what extent phenotypic variation is an ultimate response to local selection regimes and results in local adaptation; 3) to investigate whether phenotypic variation correlates with the genetic structure of populations. In the mainland landscape, where tits are assumed to disperse freely across habitat patches, little local differentiation of breeding traits has been found within a range of ca 40 km, suggesting gene swamping between populations. A molecular genetics study of populations within this landscape supported the hypothesis of a source-sink population structure with more birds immigrating from deciduous habitat patches to evergreen ones than the reverse. In a similar geographic configuration of habitats in Corsica, there was a higher phenotypic variation and a higher degree of population differentiation on a scale which is usually smaller than the dispersal range of blue tits. This difference between the mainland and Corsica is interpreted as resulting from reduced dispersal ranges of island birds and supports the divergence-with-gene-flow model of speciation.