In birds, adaptations that mitigate predators' impact are usually ineffective in confrontation with introduced and rapidly expanding invasive non-native predators. As a consequence, bird populations often decline in the period following the time when predator population is established. Changes in bird numbers and nest spatial distributions in breeding populations of the Eurasian Coot Fulica atra and Great Crested Grebe Podiceps cristatus were studied in Mazurian Lakeland, northeastern Poland in 2002–2003 and 2016, and were analyzed with reference to the abundance of invasive American Mink Neovison vison, whose density declined since mid 1990s. The study was based on a census of breeding pairs and a search of nests at 31 lakes. The numbers of breeding Coots and Grebes increased 2.6-fold and 1.2-fold, respectively. In both study periods Grebes displayed a strong tendency to nest in the vicinity of human settlements and in colonies; however, the percentage of Grebe pairs nesting near settlements and in colonies decreased from 51% to 34% and from 73% to 57%, respectively. Coots also preferred to nest in the vicinity of human settlements. Distributions of their nests have not changed significantly over time: in both periods 55–60% of Coot nests were found in close proximity to human settlements and 13–19% in Grebe colonies. The obtained results suggest that breeding populations of the Coot and Great Crested Grebe can cope with the invasive American Mink, whose predation was considered to be the main reason for waterbird declines in Mazurian Lakeland at the end of 20th century.
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Vol. 54 • No. 1