Seasonal and Long-Term Changes in the Number of Waterfowl Wintering on Lake Żarnowieckie, Northern Poland

Lake Żarnowieckie, one of the largest lakes in northern Poland, hosts a high number of waterfowl during the non-breeding season. This study presents data on bird abundance from two periods. From 1986 to 1990, coinciding with the early colonization of the lake by zebra mussels Dreissena polymorpha, counts were conducted monthly between October and April. From 2011 to 2025, only one count was performed in January. Between 1986 and 1990, the highest numbers of waterfowl were observed in November and December, with maximum counts exceeding 20,000 birds. Three most abundant species—Eurasian coot Fulica atra, tufted duck Aythya fuligula, and mallard Anas platyrhynchos—accounted for 95–98% of all waterfowl. In January, their numbers significantly decreased, remaining low in the following months. This sharp decline mainly affected diving benthivores (tufted duck and Eurasian coot), which exerted strong pressure on prey populations, leading to a rapid decrease in their numbers and forcing a shift in foraging sites. The greatest decrease occurred in seasons with the highest number of waterfowl in December, when diving benthivores were most abundant and applied the greatest pressure on benthic organisms. These seasonal changes suggest that a single January count may not fully represent the significance of the lake for waterfowl. Over the study period, both ichthyophages, the great cormorant Phalacrocorax carbo and great crested grebe Podiceps cristatus, showed substantial increases in number. For cormorants, this growth coincided with the increase in the European population, while the rise in great crested grebes likely exemplifies the establishment of a new wintering site for this species in a location rich in food and free from intense human disturbance.