Has the dwindling barn owl had a shrew too many?

ANOTHER LIFE:The balance of an ecosystem is fragile and easily thrown, especially by alien invaders

Wearing, I am told, a somewhat pained expression, its great face visibly narrowing, a well-fed owl will periodically regurgitate and drop a pellet from its beak, comprised of undigested hair and bones accumulating in its gizzard. The pellet is a hard-squeezed, egg-sized lump, often quite dry and fibrous, and spiked here and there with little white ribs and jaws. I’ve lost the one I brought back as a souvenir from Greenland. It lay below a tall boulder turned bright orange with the lichen nourished by droppings from the snowy owl’s other end – all that I saw, indeed, of that lovely Arctic predator.

Teased apart, that pellet would, no doubt, have been packed with tiny lemming skulls and fur, the one rodent to sustain the snowy’s summer prey.

A momentous surprise, however, awaited the young Irish researchers David Tosh and John Lusby parsing Irish barn-owl pellets five years ago, and it has passed their names into our modern natural history. Dissecting 10 pellets collected in autumn and winter from barn-owl roosts across south Tipperary and Limerick, they found 53 shrew skulls quite the wrong size for Ireland’s diminutive but common pygmy shrews. They belonged, unmistakably, to Europe’s greater white-toothed shrew, Crocidura russula, previously absent from Ireland and Britain.

For so many skulls to turn up, packed into owl pellets from across a wide stretch of countryside – and there were more in pellets coughed up by roosting kestrels – the little alien mammal must have been well established and spreading, possibly from pioneers introduced in hessian-wrapped rootballs of young trees imported from the Continent.

Its rapid expansion shows up in the rash of bright squares for the species, now extending south into Cork, on the interactive map of the new Atlas of Irish Mammals ( biodiversityireland.ie). Like the alien bank vole, discovered, belatedly, in Kerry in 1964 and now solidly ensconced across the south midlands and as far north as Galway, the greater white-toothed shrew (the pygmy shrew has red-tipped teeth) has been welcomed in some birding circles as extra food for birds of prey.

Lacking the field vole so common in Britain and Europe, and limited to rats, mice and shrews, Ireland has been comparatively short of food for some raptors and carnivores. But the rapid and overlapping invasion by the two new little mammals has already had a profound impact on important native wildlife.

At a head-and-body length of up to nearly 9cm, the animal is a third bigger than the Irish pygmy shrew and far more aggressive. Like the pygmy it is an insectivore, but it will sometimes attack even lizards and small rodents.

A research team headed by Prof Ian Montgomery of Queen’s University Belfast found that the presence of big shrew and bank vole is steadily reducing the abundance of wood mice and “no pygmy shrews were captured where both invasive species were present”. Indeed, further work by the university’s David Tosh has found the pygmy shrew missing from anywhere the bigger shrew is active.