Phenotypic plasticity in the expression of sexually selected traits: neglected components of variation

There are numerous studies of between-individual variation in the expressio n of sexually selected traits, but relatively few of variation within indiv iduals. We investigated within-individual variation, both between and withi n years, in the size of the sexually selected white forehead patch of the m ale collared flycatcher, Ficedula albicollis. Patch size was determined fro m direct measurements of maximum height and width and by image analysis of photographs. Measures obtained by the two methods were highly correlated wi th each other. Male patch size showed significant repeatability both within and between years, but repeatability was lower for patch width than for pa tch height. Recent reanalysis of published data suggests that sexual select ion acts mostly on the width of the patch rather than the height, suggestin g that the component of patch size subject to sexual selection is most plas tic. We found that, within individuals, forehead patch width changed in a p redictable manner within years, becoming significantly smaller later in the breeding season. This effect was more pronounced in males with an initiall y larger patch. Patch size also changed predictably between years: males wh ose parental effort we manipulated experimentally showed corresponding chan ges in patch size between years, confirming an earlier finding from this po pulation. Our results show that the size of sexually selected traits can va ry within individuals, both in response to trade-offs with other life histo ry traits, and also over short timescales. Such variation in the expression of sexually selected traits is important not only for the information it m ay provide about the processes controlling variation in such traits, but al so because failure to account for it will reduce the accuracy of any attemp t to quantify-selection on them. (C) 2001 The Association for the Study of Animal Behaviour.