The effect of weather variables on the White Stork (Ciconia ciconia) spring migration phenology

The influence of weather variables on bird migration is widely recognised as birds have been found to adjust their migration phenology under the influence of weather conditions. This is of particular interest in relation to global climate change. We investigated the long-term (1961-2000) first arrival dates (FAD) of White Storks at their breeding grounds in Lithuania and their relationship with local and regional weather variables. The return of White Storks to their breeding grounds was advanced by almost 5 days during the study period. We found that the arrival time was most strongly influenced by temperature conditions along the migration route in south-eastern Europe as well as at breeding grounds, as warmer temperatures in these areas resulted in earlier arrival of birds. Interestingly, the return of White Storks was closely associated with the onset of the 3 degrees C thermal season the date when the mean daily air temperature permanently exceeds 3 degrees C. However, in very warm years birds did not return to their breeding grounds as early as would have been expected from temperature alone, suggesting that other factors limit the further advancement of FADs. We suggest that local weather conditions, particularly the air temperature during the final stages of the spring migration, have a more pronounced effect on the arrival dates of White Storks at their breeding grounds than regional climatic phenomena (North Atlantic Oscillation or Indian Ocean Dipole).