Climatic effects on breeding and morphology: evidence for phenotypic plasticity

1. A trend towards increasing average spring temperatures has been observed in Europe over the past 25 years. This climatic variation has been attribu ted to a natural, large-scale atmospheric phenomenon, the North Atlantic Os cillation (NAO). A number of studies have reported associations between the winter NAO-index and both breeding phenology and cohort-specific morpholog y. These studies have been cross-sectional rather than longitudinal and the refore have not been able to determine whether the changes result from phen otypic plasticity or microevolutionary processes. 2. We analysed (i) cross-sectional correlations between the winter NAO-inde x and breeding performance (laying date, clutch size, fledging success and number of recruits produced) and morphological traits (tarsus and wing leng th), and (ii) within-individual variation in the same traits for individual s experiencing different values of the NAO-index in a population of collare d flycatchers (Ficedula albicollis) breeding on Gotland, Sweden, over a per iod of 16 years (1980-95). 3. None of the four measures of breeding performance changed consistently o ver the study period, while tarsus length of males (and marginally females) decreased over time. Of the six traits investigated using cross-sectional data, only laying date was related to variation in the NAO-index. 4. All characters investigated showed significant repeatability within indi viduals among years, revealing the importance of factors specific to indivi duals in determining their value. However, within individuals the NAO-index significantly affected laying date and clutch size such that females laid earlier and produced larger clutches after warmer, moister winters. 5. Our data show a high degree of concordance between cross-sectional and l ongitudinal analyses of the effect of NAO on laying date and clutch size. T he similarity of responses across and within individuals suggests that the population-level response to the NAO-index can be explained entirely as the result of phenotypic plasticity.