The abundance, diet, and prey relationships of American mink Mustela vison were studied in the Słońsk Reserve (W Poland) on two plots: shore and reservoir. Estimated mink number within the Reserve was 69 in autumn–winter 1998/1999 and 50 during spring 1999. The diet of American mink consisted mainly of mammals, birds and fish. In autumn–winter, birds formed 4–16%, whereas mammals constituted up to 56% and fish up to 62% of the biomass consumed, depending on the plot. In spring and summer, however, birds formed 45–60% of the biomass consumed in the reservoir and 35–46% of the biomass taken by mink on the shore. The European coot Fulica atra was the most frequently consumed prey. In spring, mink removed 7.8% (N=278 killed birds) of coots nesting in the Reserve, 1.8% (N=9) of breeding grebes Podiceps spp. and 11.2% of ducks (N=93 taken birds). On straw platforms only 13.6% of greylag geese Anser anser broods were successful. From 35 to 77% of the nests on straw platforms were destroyed by mink. In wooden boxes 46.4% of mallard Anas platyrhynchos and 33.3% of shelduck Tadorna tadorna nests were successful. American mink destroyed 22–40% of the nests in boxes. However, the arrival of American mink to the Słońsk Reserve has not resulted in a noteworthy decrease in waterfowl populations.
The study aimed to recognize whether the activity of a semi-aquatic invasive carnivore – the American mink Neovison vison – is related to the distribution of waterbird colonies. For this reason, we monitored mink occurrence in lake reedbeds and the fate of artificial nests imitating those of the great crested grebe Podiceps cristatus. The location of artificial nests in the grebe colony increased the probability of their survival compared to those placed outside the grebe colony. During the study, mink activity increased over time. In general, it was lower in colonies than outside of them, suggesting that the presence of natural nests does not increase the probability of mink occurrence in lake reedbeds. However, mink activity was negatively correlated with the distance from the lake shoreline and differed spatially according to the presence or absence of natural grebe nests. In grebe colonies, the probability of mink occurrence at greater distances from the lake shoreline was higher than outside, which can be explained by optimizing swimming effort while searching for prey. In conclusion, mink activity in colonies was lower than in areas with no waterbird nests, and nest location in a colony decreased predation risk by mink.