Stabilising selection on wing length in reed warblers Acrocephalus scirpaceus

The size of an animal is of utmost importance for its overall success and each species is thought to have its own optimal size. If this is true, size traits ought to be under stabilising selection unless the animal is living in a highly unstable environment. Wing length is a standard measurement of the size of birds, but up to date there have been few indications of stabilising selection on wing length. In this study we analyse recovery data for reed warblers Acrocephalus scirpaceus (n = 77 1) ringed as juveniles in Sweden from 1981 until 2001. The data showed a significant relationship between juvenile wing length and survival time, with median juvenile wing lengths (66-67 mm) being the most favourable. The estimated stabilising selection differential (C = - 0.094) supports that wing length of the reed warblers in our study is under stabilising selection. The reed warbler is a long-distance migratory species and we suggest that opposing selection pressures may act on wing length during different life history stages, and that this might explain the rather broad peak of favourable wing lengths found.