Complex phenological changes and their consequences in the breeding success of a migratory bird, the white stork C iconia ciconia

Summary The timing of bird migration has shifted in response to climate change. However, few studies have linked the potential consequences of any phenological shift on individual fitness and even fewer have disentangled the role of plasticity and microevolution in the observed shifts. The arrival date and breeding success of white storks ( C iconia ciconia ) have been recorded since the 1880s in Slovakia. We used data for two periods (1895–1913 and 1977–2007), which were considered, respectively, as populations before and after the start of climate warming. About 4000 male and 2500 female arrival dates along with 3000 breeding attempts were studied. Mean arrival dates did not differ between the two periods. During 1977–2007, males tended towards a slight delay for most fractions of arrival distribution. Protandry was reduced by 30% (1·44 days). In both sexes, the early percentiles of the arrival distribution arrived later those years with warmer temperatures at the African wintering grounds, while late percentiles advanced their arrival when temperatures were higher in the European areas flown over during migration. Mean breeding success of the Slovakian population has not changed since 1977. However, fecundity selection for arrival date reduced over the years: at the end of 1970s and 1980s, early breeders had more success than late breeders, but this seasonal trend disappeared towards the end of the study period. An early arrival and territory acquisition may have become less of an advantage due to the enhancement of feeding opportunities during the breeding season in recent decades. A century ago, stork arrival varied spatially, with earlier arrivals at low altitudes, southern slopes and warmer and drier regions. This spatial variation mostly vanished, and at present, we found little correlations with topographical and climatic gradients. We showed that long‐term temporal changes in the timing of biological events may be complex because each fraction of a population and sex may show different temporal trends in their arrival dates. In addition, the effect of biotic and abiotic factors may change consistently in space and time, and thereby phenotypes' value depends on the circumstances that are expressed due to its variable fitness consequences.