Sad silence as mink moves in

Out on the wide green lawn of the duach the other morning, a naturalist friend spotted a small, black animal rippling across the sward with a couple of hooded crows in close and raucous pursuit. It's good to know we have something in nature prepared to give the mink a hard time, but sad to have this blatant confirmation of its presence in Thallabawn. This ribbon of coast beyond the mountains is a final westward frontier in the animal's 50-year spread across the island. There have, of course, been ominous signs. There was silence this summer at Cross Lough, a coastal lake to the north which usually echoes to the cries of sandwich terns, nesting wing to wing on the little stone raft of a crannog. These birds are notoriously nervous of predators, and quick to shift their breeding ground, so we have probably lost them for good.

And on our own swan lake, the resident pair of mutes are unusually and sadly childless. Only a couple of years ago they led a fluffy flotilla of eight cygnets into the autumn. Their lack of success this summer is pointed up by the family groups in the current gathering of incoming swans, mutes and whoopers. Small cygnets are just one of the waterbirds at risk from mink: moorhens, ducks and coots are all on the list of prey.

This is the time of year when mink are most noticed, as young ones disperse and make their impact on wildlife. Hooded crows are great harriers of mammal predators, in or out of the breeding season: I have watched a pair mobbing a fox along exactly the same stretch of shore. And when the mink first appears, country people, especially anglers and shooters, are quick to share a feeling of invasion and to brood on the implications.

In the face of such a determined and agile little killer, it's hard to know just how philosophical to be. Biologists assure us that, after three or four years of mayhem in the initial surge of colonisation, mink space themselves out to match their food supply and a new equilibrium emerges.

Something similar could happen, after all, as the pine marten increases its numbers and range, with full protection from the wildlife law. The marten (a mustelid, like the mink) is a long-persecuted native, once reduced to a last stronghold in the Burren but now filtering out from there and other refuges. Most people would be glad to see it spreading through Ireland's treetops, preferring not to think about its appetite for songbirds.

The otter is another mustelid, and among the early fears attending the escapes from mink farms was that the American alien would supplant the otter in river habitats. But this has not been happening: indeed, on some Irish rivers and lakes, the droppings of mink and otters can be found almost side by side.

On inland rivers and lakes, the two animals seem to have shared out the available prey on fairly equable terms. The otter, with its superior speed and underwater vision, takes the bigger, faster fish, while the mink makes do with perch, eels and crayfish and a wide range of other food: frogs and rats as well as waterbirds.

At the west coast, probably the otter's most secure stronghold in Europe, the mink again picks a different niche, taking crabs and fish from rock-pools and finding more of its food on land (in sand-dune rabbit warrens, for example). But the mink can swim across at least 3 kilometres of sea, and, hopping between islands, can devastate colonies of nesting gulls and terns. In one recent 10-year study in the west of Scotland, common terns declined by one-third and black-headed gulls were down by half.

Wildlife adapts, in time, to the presence of a new predator. Seabirds will move to less accessible islands and our inland waterbirds, through selection, will build in safer places. There will be more repeat clutches: mallard, for example, can extend their breeding season and lay another dozen eggs to replace those the mink has raided.

The animal's presence does, however, add to the pressures, already severe, on Ireland's breeding waders such as snipe and lapwing. As areas of wetland and rough grassland dwindle in the farming landscape, waders are being pushed into nesting on river-banks, right on the mink's doorstep.

With a carnivore so casual in its appetite, it is hard to tell which species is likely to be most threatened by Mustela vison. In Britain, for example, the most alarming impact of the mink has been on the water vole - "Ratty" in Wind of the Willows. In some isolated are as, it has actually been exterminated.

Ireland doesn't have water voles. Britain, on the other hand, doesn't have the abundance of crayfish, which thrive in our rich, slow, limestone rivers and supply the bulk of the mink's food in the midlands. Just as the badger may hoover up earthworms, yet consume, at most, a mere five per cent of the local annual worm production, the output of crayfish is well able to support the mink.

The west coast, however, is not crayfish country, and the mink's random appetite could have a lamentable impact in some unexpected directions. I think, for example, of the dipper, that strange and fascinating bird with a white bib that lives on fastflowing streams and "walks" underwater in search of insects. Some books show it as a bird of the uplands, but I watch it in little rivers right down to the shore.

I rarely see more than one to a long stretch of stream, though dippers, of course, do pair off and even sing to each other, loud as wrens, in early spring. They nest, very often, in a cavity under a bridge, a few feet up from the water - but probably, now, not high enough. Dippers will survive, but perhaps be forced to retreat to the high ground after all - just one of the new patterns that the mink's advance will spread through many species, like ripples in a pool.