Abstract: The census of non-breeding bird assemblages using the artificial water bodies can provide valuable insight into the impact of locality and climatic variation on the seasonal and inter-seasonal dynamics of bird species. To assess the basic structural characteristics of the bird assemblage, we monitored waterbirds over ten non-breeding seasons (2013/2014–2022/2023), at the Krpeľany water reservoir in northern Slovakia. A total of 39 water bird species were recorded, with a mean abundance of 771 birds per survey. Five dominant species (Mallard Anas platyrhynchos, Tufted Duck Aythya fuligula, Coot Fulica atra, Great Cormorant Phalacrocorax carbo and Mute Swan Cygnus olor) accounted for more than 80% of all observed birds. Species richness remained relatively stable throughout the study period (mean coefficient of fluctuation = 2.03; 39 species). However, the number of birds varied significantly both between seasons and within seasons. The highest numbers were recorded during autumn and early winter, followed by a gradual decline in the later part of the season. The total number of birds in 2016–2023 had decreased to nearly half of the levels observed during the initial seasons (2013–2015). This decline might be attributed to the development of new water bodies in the surrounding area. The Krpeľany water reservoir can be regarded as a regionally important site for non-breeding water birds. However, it is increasingly affected by sedimentation, which may cause serious ecological and commercial challenges.

Key words: wintering waterbirds monitoring, abundance, dominance, population trends, Slovakia