A revealing sign of mink's impact comes from outwardly tranquil lake islands devoid of human population.
Steven Newton, who analyses survey data for Birdwatch, regards Common Scoter (inset right), a sea duck breeding on these sanctuaries, as highly endangered. The all-black male has an orange beak, while the female is brown. "It nests mostly on Lough Corrib and Lough Ree where there are about 100 pairs only, but it is declining severely," he says.
Local reports suggest breeding mallards on Lough Corrib have declined from moe than 150 pairs in the 1970s to just 10 or 15 today. Mink traps are now being set around Corrib. The predator itself is seldom seen - it hunts largely at night and spends long periods resting.
Newton sees a telltale pointer to predator activity: the females - literally sitting ducks on the nests - are disappearing first. "Predators are taking a lot of females. What we would notice is the sex ratio starting to change, say from 20 male and 20 female to 20 male and 15 females. That would be the case on Lough Conn and Lough Cullin in Mayo."
Common Scoter on these lakes were reduced to just two or three females and some 20 males the year before last. "We think they will probably disappear there this year," Newton predicts darkly.
The boomerang-shaped Lough Erne, near Enniskillen, has dozens of islands once rich in nesting sites. Studies show a dramatic decline, and falling ratios of females to males. Foxes, though present locally, cannot reach freshwater lake island nests. Recent declines are too steep to have arisen from long-present lesser black backed gulls killing chicks. Erne has now lost all its Common Scoters.
"Trapping by RSPB staff shows mink regularly visit islands used by nesting ducks and points strongly to predation of incubating birds," says James Robertson of the Royal Society for the Protection of Birds (RSPB) in Northern Ireland. "Besides scoter, we're talking about big declines among breeding waders - lapwing, snipe, redshank, coots, moorhen, with almost two thirds of those populations disappearing in the last 10 years or so," he says. Compared with its larger relation, the otter, which eats mostly fish, mink are seen as more likely to feed regularly on birds.