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 w as 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 t he number of chicks hatched per pair of coots, as well as the average numbe r of nests per pair of moorhens and the percentage of moorhen clutches hatc hed. 3. Mink diet during the birds' breeding season (March-September) was studie d through seat 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 energ y 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, nest ing behaviour by coots make them very vulnerable to mink predation. We hypo thesize that the persistence of coot populations in areas with high mink de nsity requires immigration from surrounding populations with lower mink imp act.