Various studies have explored the relationship between atmospheric conditions on periodic cycles of bird migration, however, there are limited knowledge on how weather condition really affect birds migration on an individual fitness level. The informations gathered were not sufficient to describe the birds internal self-sustained circadian rhythm and the external signal to guarantee suitable departure and arrival timing of migration. This research elaborated on the factors contributing to the timing of migration by using a long-term statistical dataset covering longitudinal range in northwestern Europe. The environment conditions including: spring vegetation in Europe, local spring temperature in northern Europe and the North Atlantic Oscillation index.
The phenological data were derived from spring data on migratory passerines from various remote islands: Heligoland in the North Sea, Christians, and Jury in the Baltic Sea. A standard of 20 birds captured and observed per season at each ringing station for at least 12 years were incorporated in the research. Various species were excluded from the study due to segregation migration route. These specification narrowed down 12 species which six of these are short-distance migrants and the other six are long-distance migrants.
The environmental data consist of monthly normalized difference vegetation index (NDVI), an average monthly temperature of local weather station, and the winter index. The NDVI demonstrate an approximation of ecological condition between regions and years. By calculating the average of different weather stations and take the mean of each of the month of spring, an overall provincial temperatures for the months can be calculated. This was necessarily due to the limited knowledge of individual fitness. NAO was favored because its correspond with the temperature, precipitation, and air current during winter.
Winter NAO became a guideline to separated the migratory birds into five groups corresponding to the distance of migration. Additionally, species-specific migration phase model was established using the environment measurement versus the time that migration occur for spring NDVI and spring temperature. The model incorporate all three climatic variables (spring NDVI, spring temperature, winter NAO) provided a rough evaluation on the general differences of migration phenology. The explanatory power, r2, of each model will then be compared with other model to inspect the changes in stages of migration and migrating group due to changes in weather condition. R2 calculated to analyzes the relationship between all three climatic...