The impact of American mink Mustela vison on water birds in the upper Thames

1. The effect of mink predation on water birds during the breeding season was studied between March and September 1996 in a 33-km long stretch of the upper Thames river, England. 2. Mink presence significantly affected the density of breeding coots and the number of chicks hatched per pair of coots, as well as the average number of nests per pair of moorhens and the percentage of moorhen clutches hatched. 3. Mink diet during the birds' breeding season (March-September) was studied through scat analysis. Ralliformes (coots or moorhens) represented 10% of the ingested biomass and were the fourth prey in importance after rabbits (45%), fish (25%) and small mammals (14%). Mink obtained 11% of their energy requirements from coots and moorhens. 4. Impact of predation by mink during the bird breeding season was moderate to high for moorhens (16-27% of adults and 46-79% of broods) and high for coots (30-51% of adults and 50-86% of broods). 5. Although moorhens seem well adapted to withstand predation by mink, nesting behaviour by coots make them very vulnerable to mink predation. We hypothesize that the persistence of coot populations in areas with high mink density requires immigration from surrounding populations with lower mink impact.