Changes in numbers and distribution of wintering Long-tailed Duck Clangula hyemalis in Baltic offshore waters

Regular midwinter counts of waterfowl (included in theInternational Midwinter Counts, IWC, organized by whatis now Wetlands International) has been undertaken inSweden since the start in 1967 and are still going on,now as a part of the National Environmental MonitoringProgramme. After the first exploratory years includingcountry-wide surveys in 1971–1973 undertaken bothfrom the ground, the air and boats, the programme wasstandardized to cover an adequate sample of coastal andinland sites of south Sweden for the production of annualmidwinter population indices for important species(areas further north in Sweden are normally ice-coveredand without importance for waterbirds in winter). Furtherlarge scale surveys were undertaken in 1987–1989, 1993and 2004 (country-wide survey). The programme did notcover the seaducks in offshore areas (later included in aspecial programme). The present contribution analysesthe first 40 years of midwinter counts in Sweden, especiallywith the aim to elucidate changes in distributionand numbers of the different species during this time series.During the period there was a marked change in thewinter weather and especially the ice situation in Swedenin January, being a factor of importance for the winteringwaterbirds. During the years 1967–1987 there werefive really hard ice-winters, whereas in the latter part ofthe period there was no really hard ice-winter. Comparisonsbetween the country-wide surveys in 1971 and 2004showed a marked increase from 172,000 to 627,000 individualsexcluding the offshore species Clangula hyemalisand Melanitta spp. Anas platyrhynchos increasedfrom 67,000 to 150,000, Aythya fuligula from 54,000 to230,000 and Bucephala clangula from 19,000 to 75,000.Increases were also found in most of the other species.In addition, Anas penelope and Podiceps cristatus establishednew wintering traditions in south Sweden. Midwinterindices for nine out of ten species showed significantlyincreasing trends over the 40 years, no speciesdecreasing. In Anas platyrhynchos and Aythya fuligulathe indices showed fluctuations around a steady level,then a marked increase followed after the last cold winter,whereas species like Bucephala clangula and Mergusserrator showed increasing trends from the start ofthe counts. Fulica atra crashed markedly after one coldwinter and remained for years on a low level before recoveringbut the overall trend was increasing. To a largeextent the increases found are probably due to the milderwinters in later years, which also led to changes in winterdistribution within the country, also being reflected in theregional totals for different species. Part of the increasesdoes also reflect genuine population increases as show bycomparisons with IWC results from other countries