Has the dwindling barn owl had a shrew too many?

Sat, Nov 24, 2012, 00:00

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.

This has become of urgent concern to BirdWatch Ireland’s raptor conservation project, in which Tosh works with John Lusby, a postgraduate researcher at NUI Cork and now the organisation’s raptor-conservation officer.

The barn owl has suffered steady decline in Ireland, its numbers falling by more than half since the 1970s. The conservation team confirmed 133 nest sites last spring, but the national population is now put at 450 to 500 pairs, most of them in the warmer southern counties and a quarter of them in Cork.

Originally it was thought that lack of nest sites was limiting the population, and 250 nest boxes have been installed in recent years in empty barns, castle ruins and other classic locations. However, surveys have found plenty of potential nest and roosting places in many parts of Ireland. And while the known nest sites are the most the research team have located during the project, this year’s rain-soaked breeding season was a bad one for the owls.

Of 80 nests the project monitored, a quarter failed to fledge any young. While a lot of heavy rain can keep the owls from flying, a high rate of mortality and emaciated chicks have marked the productivity of the past three years, especially at nests in south Tipperary. Here the owls have been feeding almost exclusively on greater white-toothed shrews. Is it the lack of pygmy shrews, a food to which the Irish owls are long adapted, that is lowering their breeding success? Research will continue with this in mind.

Montgomery has warned that the changes among small mammals, as the invaders spread, “are likely to reverberate throughout the ecosystem”, changing vegetation, insect communities and the productivity of bird and carnivore predators. The impact on the barn owl could prove the first bitter surprise. Far from improving the barn owl’s diet, the greater white-toothed shrew may have wrecked it.

Eye on Nature Your observations and questions

I have seen a briar growing from the fork in an ash tree, about 15ft up. It is growing downwards, and the tip is now a few feet from the ground. Was it grown from a blackberry dropped by a bird?

C Guckian Dromahair, Co Leitrim

More likely from a seed passed by a bird, which took root in detritus in the fork of the tree.

I saw a blue tit take whole nuts from my feeding baskets and hide them in my hanging baskets under leaves.

Siobhán Hearne Waterford

Some blue tits hoard food, as do some other members of the tit family.

I attach photos of two holes that have appeared, one under the path beside the dog kennel. The other is 13cm in diameter. Both go down about 30cm.

Paul Kerr Barna, Co Galway

They are possibly exploratory rabbit holes.

We seem to have a fox with a fancy for corn on the cob, judging by fox scats with almost entirely undigested corn. The nearest maize field is almost a kilometre away. The fox comes every year for pear windfalls.

Michael McCarthy Bagnelstown, Co Carlow

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