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.