Another Life: Upheavals in the furry underworld

The sight from our breakfast window of three or four hares pretending to ignore each other in the rocky meadow up the hill has promised that some mammal affairs, at least, are proceeding on cue. Events in Ireland’s mammal underworld, however, where small furry creatures rustle through grass and leaf litter, seem less predictable. Major changes have been set in train by an invasive little vole and an alien shrew, with consequences for the barn owl, a declining Irish bird.

Scientists studying barn-owl diet tease apart its pellets – furry, bony, feathery balls of indigestible remains. Owls usually swallow their prey whole, or in large chunks, and cough up the pellet about six hours later. In Greenland once, I collected a snowy owl’s pellet from beneath a great boulder long used as a perch for hunting and swallowing. It was full of lemming bones and fur – the nearest I got to actually seeing the owl.

For a research team at University College Cork, however, a trove of barn-owl pellets was a mass of material, weighing 12.5kg, recovered in 2013 from a barrel that had fallen from a tree near Leap, in west Cork. Installed as an artificial nest site in 1994, it also contained five barn-owl skeletons of various sizes.

A group of "citizen scientists" was then assembled, including schoolchildren and former students from the Cape Clear ecology courses run by Geoffrey Oliver and the UCC mammalogist Paddy Sleeman. As the pellets were sieved under running water, thousands of fragments for tweezers-and-microscope identification needed a widely expert team – there were more than 6,000 from different vertebrate prey, including many birds.

Almost half the fragments came from bank voles, Myodes glareolus, with much smaller shares from field mice, brown rats and pygmy shrews. There were feathers and bones of 149 birds, including blackbirds, house sparrows, skylarks and woodcock, along with seven bats, a few fish bones, and lots of bits from beetles.

Barn owls in Britain feed mainly on field voles, a species that never reached Ireland. The range of the alien bank vole has spread widely across Ireland since its discovery near Listowel, in Co Kerry, in 1964. It had almost 40 years of Irish residence before that, having arrived with German machinery for the Ardnacrusha power project in the 1920s (a theory supported by DNA studies).

Bank voles mostly prefer woodland and hedgerow habitats, yet they now inhabit the southwestern third of the island, from the Co Cork coast to mid Co Mayo. Other Irish studies of barn-owl pellets show increasing reliance on the vole as prey, rather than the field mice and rats that used to dominate. The owl's long legs reach easily into dense cover, but in a paper to be published in Irish Birds, the research journal of BirdWatch Ireland, the UCC scientists, led by Dr Sleeman, consider if the vole is spreading into open country, to take on the wider role that the field vole has in Britain.

Meanwhile, in 2008, barn owls in part of Co Tipperary were found to be feeding almost exclusively on the alien greater white-toothed shrew, Crocidura russula. This small but voracious invader, which probably arrived in Ireland more than a decade earlier, now covers the heartland of the bank-vole area. While the voles have been gradually replacing the field mouse, the voracious alien shrew has been displacing the smaller native pygmy shrew. Where both invaders coincide, neither native mouse nor shrew can persist.

Evidence for this dramatic change was offered in a major paper by biologists at Queen's University Belfast led by Prof Ian Montgomery ('Invasional Meltdown': Evidence for Unexpected Consequences and Cumulative Impacts of Multispecies Invasions. Biological Invasions, 2012). The impact, it says, is likely to reverberate throughout the ecosystem. "Vegetation composition and structure, invertebrate communities and the productivity of avian and mammalian predators are likely to be affected."

The last bird atlas for these islands showed continuing decline in the Irish breeding range of the barn owl – more than a third in 40 years. Will a change of diet help its fortunes? Probably not, where it flies low across busy roads or in the sprawl of urban concrete. And there's a new and unexpected risk from yet another potential invader: Britain's tawny owl, Strix aluco.

Seen hitherto as a highly stay-at-home species (rather like the woodpecker), the tawny owl has been growing its UK population and beginning to fly west, recently breeding for the first time in the Isle of Man. In 2013 the owl turned up in Co Down, not far from Belfast, and there have been unconfirmed reports from Co Kerry.

The tawny owl now figures among potential invasive species: its attendant risk was assessed in 2014 by the National Biodiversity Data Centre. "Because of the adaptive nature of the tawny owl," it concluded, "both in habitat and nesting requirements, the potential establishment of this species in Ireland may negatively impact upon the sensitive barn owl population."

Michael Viney's Reflections on Another Life, a selection of columns from the past four decades, is available from irishtimes.com/irishtimesbooks