Phenological response to climate change and population trends of migratory birds

The recent increase in temperatures across the Northern Hemisphere is causing marked changes in the phenology of plants and animals, which shows a trend towards earlier start of spring events, such as flowering and leaf emergence, or earlier onset of breeding activities. Migratory birds make no exception, and there are several studies documenting advances in timing of spring migration in recent decades. However, not all species have advanced their migration timing, and this may have lead to a mismatch between timing of reproduction of these species and the peak seasonal availability of their main food sources. In fact, organism belonging to different trophic levels, such as e.g. insectivorous birds and their invertebrate prey, may have different abilities to show a phenological response to rapid climate change, and prey could advance phenology more than predators. Importantly, mismatched species could suffer from reduced reproductive output, leading to population declines. We have reviewed the long-term changes in timing of spring migration among European bird species in the period 1960-2006, and - besides confirming a general pattern of advance of migration dates - found that such changes varied significantly among species, different populations of the same species consistently showing an advance, whereas populations of other species did not change or even delayed their phenology. We then analysed whether the phenological response to climate change, as assessed by long-term trend in spring migration timing, predicted population trends, assuming that species that did not advance timing of migration could be more mismatched compared to their food sources, and thus be characterized by declining populations. Indeed, we found that species that did not advance timing of spring migration suffered stronger population declines in the period 1990-2000 compared to species which advanced their timing of spring migration, whose populations were instead stable or increasing. Therefore, failure to adjust timing of spring migration to climate change may increase the chances of extinction of migratory bird species in the near future.