Why are bird migration dates shifting? A review of weather and climate effects on avian migratory phenology

Many studies have reported statistically significant associations between bird migratory phenology and climatic variables, and, consequently, it is mostly accepted that recent shifts in migration dates are a reaction to present climate change. Impacts of weather and climate on departure date, progression and stopover frequency and duration have been reviewed in order to explain the current knowledge of climatic mechanisms underlying such phenological shifts. Climate in departure areas can affect population phenology via the size of the returning population. In a short-term sense, ecological conditions can affect acquisition of migratory body condition during the days preceding departure. Migratory phenology can also be affected by quality and quantity of replaced feathers during moult. Adverse weather conditions en route strongly hinder the progression of individuals and even force them to land. The time spent on stopovers will greatly depend on the ecological conditions there and the opportunities for refuelling. Temperature is by far the climatic variable most frequently related to avian phenology. The use of climatic indices is spreading because they have the advantage of synthesizing weather conditions into a single variable. Remote sensing is probably the best option to explore ecological conditions in areas used by migratory species. Most of the climatic variables employed are from the arrival area, while climate from passage or departure areas is rarely assessed. The overwhelming majority of studies have used variables defined by months, while the use of periods designed ad hoc according to species' biology is almost anecdotal. It can be concluded that further research is needed to disentangle the true relevance of each type of climatic variable over avian migratory phenology during each phase of migration.