Łukasz Krajewski
Ornis Polonica 2018, 59: 250–263
https://doi.org/10.12657/ornis.2018.4.1
Abstract: A census of Garganeys Spatula querquedula and Northern Shovelers S. clypeatawas carried out in 1–11.05.2018 in the Natura 2000 site Ostoja Biebrzańska PLB200006 on 41 sampling plots of 1 km2, randomly selected from two layers – optimal (21 plots) and suboptimal habitats (20). A total of 117 males of Garganey and 95 males of Northern Shoveler were found, including 107 and 77 males, respectively, classified as IBP (indicative of breeding pairs). The number of Garganeys in the Biebrza Marshes was estimated at 760 males, and the number of Northern Shoveler at 490 males. Observed changes in the social structure between the first decade of April and the first decade of June suggest that the first decade of May is the optimal date for counting of these species. In comparison with 1976–1980 the numbers of Garganey have decreased by about 60%, while no declining trend has been observed for the Northern Shoveler. The decrease in the number of Garganey and the stable number of Northern Shoveler are in line with their pan-European population trends. The decline in the Garganey population is most likely due to factors operating on African wintering grounds, and to a lesser extent, factors occurring on the breeding grounds, such as the declining trend in the Biebrza spring floodwaters associated with climate warming and the abandonment of grazing and mowing of riverside meadows.
Keywords: Biebrza river, Garganey, Northern Shoveler, population trends, river valleys
