From 1967 until 1990, populations of five Sylvia species migrating through the southwestern coast of the Baltic Sea were considerably low and significantly decreasing in the Blackcap (Sylvia atricapilla), Garden Warbler (S. borin) and Barred Warbler (S. nisoria). While after that period a rapid increase in the populations of these migratory species was noted. Similar changes that had started about 1991 were shown by all five Sylvia species. Such coincidence could indicate some common factors acting upon the studied group of migrants at the same time. Apart from a clear increase in the long-term dynamics of the species observed at Bukowo-Kopań station, the Pearson correlation coefficients of these dynamics calculated for every pair of the analysed Sylvia species in 1967-2006 were high and statistically significant.
Based on the distribution of recoveries of the Lesser Whitethroat (S. curruca) we assumed that the main fraction of birds passing the south-western coast of the Baltic Sea originated from the Scandinavian Peninsula. Recent climate changes observed in this area were consistent with the time when the increase in the number of caught birds was noted. Consequently, we suspect that the analysed Sylvia species passing this part of the coast share vast breeding grounds located mainly on the Fennoscandian Peninsula, where their populations are affected by the same common factors.
