• Ficedula albicollis;
  • morphology;
  • NAO-index;
  • phenotypic plasticity;
  • timing of breeding


1. A trend towards increasing average spring temperatures has been observed in Europe over the past 25 years. This climatic variation has been attributed to a natural, large-scale atmospheric phenomenon, the North Atlantic Oscillation (NAO). A number of studies have reported associations between the winter NAO-index and both breeding phenology and cohort-specific morphology. These studies have been cross-sectional rather than longitudinal and therefore have not been able to determine whether the changes result from phenotypic plasticity or microevolutionary processes.

2. We analysed (i) cross-sectional correlations between the winter NAO-index and breeding performance (laying date, clutch size, fledging success and number of recruits produced) and morphological traits (tarsus and wing length), and (ii) within-individual variation in the same traits for individuals experiencing different values of the NAO-index in a population of collared flycatchers (Ficedula albicollis) breeding on Gotland, Sweden, over a period of 16 years (1980–95).

3. None of the four measures of breeding performance changed consistently over 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 individuals among years, revealing the importance of factors specific to individuals 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 longitudinal analyses of the effect of NAO on laying date and clutch size. The 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.