Analyses of long-term data indicate that human-caused climatic changes are affecting bird phenology in directions consistent with theoretical predictions. Here, we report on recent trends in the timing of spring arrival and egg laying found within a western European Pied Flycatcher Ficedula hypoleuca population. Mean egg laying date has advanced over the past 20 years in this population. The advancement in egg laying date was stronger than the advancement of spring arrival, suggesting that Pied Flycatchers are changing these stages of their annual cycle at different rates. It could be shown that selection for earlier breeding had increased. Hence, the observed adjustment in laying date did not match the advancement of spring. Our findings raise general questions about the adaptability of migratory birds to rapid environmental changes. Adaptive advancement of reproduction in response to increasing spring temperatures and to the concomitant advancement of food supply could be held back, because annual breeding and migration cycles are controlled primarily by endogenous rhythms and photoperiodic cues which do not relate to temperature. Migrants may have several options for arriving earlier on the breeding grounds, including an increase in migration speed, earlier departure from the wintering area or a shortening of migration distance. Changes in migratory behaviour could be accomplished either by phenotypic plasticity or by selection on different genotypes. Although descriptive field data provide compelling evidence for changes in, and possible constraints on, the timing of breeding and migration, their explanatory power in predicting the limits of adaptation remains restricted. We review recent experimental approaches, which explicitly test the relative roles of genetic versus environmental factors in the adaptation of life-cycle timing to global environmental changes.
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Vol. 55 • No. 1–2