Population fluctuations and timing of spring migration of the Scandinavian Bluethroat Luscinia svecica svecica at Ottenby Bird Observatory, Sweden, 1955-2008

Abstract

In this study, 54 years (1955-2008) of consecutive trapping data from Ottenby Bird Observatory on the island of Oland, SE Sweden, was used to analyze the spring passage of the Scandinavian subspecies of the Bluethroat Luscinia svecica svecica. The aim was to investigate trends in the numbers of Bluethroat passing this site and to provide statistics related to the phenology of migration. Trapping of Bluethroats at Ottenby may be seen as an index of population numbers in the recruitment area, especially for the latest decades when trapping conditions have been standardized. The number of trapped individuals was stable both in the long and short term, but median spring passage has become significantly earlier over the study period. The spring migration of the spe-cies showed clear age and sex related differences in timing. Male Bluethroats preceded females with about three days, and adult birds preceded juveniles of both sexes. Finally, the local weather during the peak passage significantly affected the number of trapped individuals, with the largest number trapped in days with head winds from the northwest sector.