| Summary: | There are numerous studies of between-individual variation in the expression of sexually selected traits, but relatively few of variation within individuals. We investigated within-individual variation, both between and within years, in the size of the sexually selected white forehead patch of the male collared flycatcher, Ficedula albicollis. Patch size was determined from direct measurements of maximum height and width and by image analysis of photographs. Measures obtained by the two methods were highly correlated with each other. Male patch size showed significant repeatability both within and between years, but repeatability was' lower for patch width than for patch height. Recent reanalysis of published data suggests that sexual selection acts mostly on the width of the patch rather than the height, suggesting that the component of patch size subject to sexual selection is most plastic. We found that, within individuals, forehead patch width changed in a predictable manner within years, becoming significantly smaller later in the breeding season. This effect was more pronounced in males with an initially larger patch. Patch size also changed predictably between years: males whose parental effort we manipulated experimentally showed corresponding changes in patch size between years, confirming an earlier finding from this population. Our results show that the size of sexually selected traits can vary within individuals, both in response to trade-offs with other life history traits, and also over short timescales. Such variation in the expression of sexually selected traits is important not only for the information it may provide about the processes controlling variation in such traits, but also because failure to account for it will reduce the accuracy of any attempt to quantify selection on them. © 2001 The Association for the Study of Animal Behaviour.
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