Monitoring of numbers in passage populations of passerines over 42 years (1958-1999) on the Courish Spit of the Baltic Sea

Sokolov, L.V., Yefremov, V.D., Markovets, M.Yu., Shapoval, A.P. & Shumakov, M.E. (2000): Monitoring of numbers of passage populations of passerines over 42 years (1958-1999) on the Courish Spit of the Baltic Sea. Avian Ecol. Behav. 4: 31-53. The analysis of trends in numbers of passage populations in autumn in 24 passerine species on the Courish Spit on the Baltic Sea recorded by trapping in large Rybachy-type traps showed a significant increase in the major- ity of species in the 1960s and 1980s and a decline in the 1970s and 1990s. In ten out of 12 species under con- sideration we found a significant positive relationship between number estimates of local juveniles during the post-fledging period on the Courish Spit and numbers of autumn migrants. In nine out of 12 species a signifi- cant negative relationship was recorded between the numbers of juveniles on autumn migration and the timing of breeding on the Courish Spit. A test for relationship between the mean monthly temperatures in spring and in summer in our area and numbers of autumn migrants revealed a positive correlation of numbers with mean April temperature in 11 species. We conclude therefore that the long-term number dynamics of autumn pas- serine migrants on the Courish Spit is a good estimate of the trends in breeding populations that migrate over our study site in autumn. Long-term fluctuations in the numbers of passerines in the Baltic region are in our opinion primarily related to the global climate change in Europe in the 20th century. Considerable increases in numbers in 1960s and 1980s are most likely explained by warm springs that were frequent in these decades. High spring air temperature caused earlier arrival and breeding, and also higher breeding success and offspring survival, which has led to an increase in breeding and passage populations in these decades.