Some of the factors that could influence changes in phenology of the song thrush (Turdus philomelos) during spring migration have been analyzed in relation to the timing of their return to the south-east Baltic region over the last 40 years. These include wind direction and velocity, temperature and precipitation in the wintering areas and along the migratory route. In March, a significant correlation was found between the timing of passage in the Baltic region and both the air temperatures in wintering area and winds over the migratory route. In April, when mass migration of thrushes occurs, the timing of passage was correlated with temperature, winds and, partly, precipitation over the route of migration. In this month, the frequency of tailwinds increased significantly along the route of migration from south-west France to the Baltic region over the last 40 years. The tailwind frequency was correlated with the beginning, middle and end of spring passage, accounting for 51% of the variance of median dates of the passage. The higher ambient temperatures over the migratory route explain nearly 20% of the variance in the timing of passage. Our results suggest that the advance in the timing of spring migration is primarily due to (1) the increased frequency of tailwinds favorable for migratory flights over the migratory route, and (2) the earlier emergence of the conditions enhancing migration due to higher spring temperatures in the winter quarters and on the migratory route.