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Invasive alien predators pose a threat to native fauna and the studies of their feeding habits are crucial to understanding their impact on prey populations. Diet of the American mink Neovison vison, an invasive species in Europe, is relatively well studied based on scat analysis, however, the use of other methods of diet analysis enables a better overview of this issue. We analyzed the isotopic composition of carbon and nitrogen in the livers, which reflects the diet from about 30-40 days (scat analysis provides information on the diet over 1-2 days only) of the American mink from four national parks in Poland and in the muscles of three types of mink prey (root voles, common frogs, and roach) in order to estimate their contribution to the mink diet. Mink in Biebrza and Narew National Parks fed mainly on frogs and fish, in Drawa National Park on voles and fish, and in Warta Mouth National Park almost exclusively on fish-as shown by Bayesian mixing models calculated for three selected groups of prey. There was no isotopic evidence for differences between the diets of male and female mink. In all groups of prey, we found surprisingly high differences between individuals of the same species from different study sites in the isotopic composition of both δ 13 C (up to 3‰) and δ 15 N (up to 6‰). Based on a detailed literature review, we predict that the main reasons for these variations are differences in abiotic environment, food availability, and trophic position. We also indicate a lack of data on trophic discrimination in fish and amphibians, which makes it hard to assess the influence of differences in trophic position on isotopic variations. We suggest caution to authors who plan to study geographical variations in diet of animals using stable isotope analysis without acknowledging that taxonomically and ecologically similar prey can differ in isotopic composition between studied areas.

Colonial breeding in birds provides protection from predators and may be particularly important when birds have to cope with an invasive predator. The probability of nest predation in a colony can vary depending on several factors, such as the nest’s location in the colony and the level of aggregation of nests. We studied the nesting success of colonial great crested grebes and monitored the occurrence of the non-native invasive American mink in the colony. From among 92 grebe nests, 54.3% were successful. The daily survival rate (DSR) of grebe nests was positively affected by the increasing distance between the nest and lake shoreline, and negatively affected by the increasing distance between the nest and the five nearest grebe nests. The probability of mink occurrence in the colony increased with consecutive days of the breeding season and decreased with increasing distance from the lake shoreline. The DSR of grebe nests decreased with the increasing probability of mink occurrence along the shoreline distance gradient and the day of the breeding season. The results of the study confirm the impact of the American mink on waterbirds during the breeding season but also indicate that large breeding colonies are partially safe from mink predation, and that nest accessibility and the dilution effect influence the probability of nest survival. Our data suggest that the limited access to safe breeding sites on large lakes that can supply adult grebes and their chicks with food may affect bird productivity and population numbers at the landscape level.Electronic supplementary materialThe online version of this article (10.1007/s00442-018-4270-8) contains supplementary material, which is available to authorized users.

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