Environment

The bank vole was introduced in the 1920s with machinery imported from Germany

Bank voles have become important as prey for Ireland’s barn owl population

The bank vole, such as this one in Co Kerry, was previously found only in Munster. Photograph: Valerie O'Sullivan
The bank vole, such as this one in Co Kerry, was previously found only in Munster. Photograph: Valerie O'Sullivan
Richard Nairn
Sat Jan 04 2025 - 06:00

Just the other day our tomcat brought in what looked like another mouse, a regular casualty of his nightly hunting expeditions. When I scared him off, he dropped his prize at my feet and I knew right away that it was no mouse as the tail was short and it had a reddish tinge to the fur on its back and a blunt, rounded head. Having consulted my identification guides, I concluded that it was a bank vole.

This was a revelation as this small mammal was previously found only in Munster and is still the only known vole species from Ireland.

The bank vole was first discovered in Ireland as recently as 1964 in Co Kerry. There is good evidence from genetic studies that it was introduced in the 1920s with machinery imported from Germany for the building of the Ardnacrusha hydroelectric station, upstream of Limerick, and it had been breeding in Munster since then.

After a slow start the species spread across a number of counties at a rate of about 1.5km per year, fastest in areas of dense vegetation. Quite quickly it became an important prey item for several birds of prey, notably the kestrel and the barn owl. These birds represent flagship species for farmland conservation. Barn owls (often called “white owls” locally) are fascinating birds that may breed surprisingly close to people but can suffer seriously as a result. Reduction of hunting areas due to agricultural intensification, road collision casualties, poisoning by rodenticides and the loss of nest sites in old buildings are just some of the pressures on them.

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In 2008 John Lusby of BirdWatch Ireland initiated the Barn Owl Survey and began to undertake organised surveying and recording of the species. His colleague Michael O’Clery’s interest in barn owls was stimulated when he discovered a dead owl by a derelict building near Caherciveen, Co Kerry. O’Clery subsequently went to see a known nest site at Milltown, Co Kerry. He remembers his excitement on this visit.

“I watched a pair emerging from a hole, high on the gable wall of the old building and I was hooked. I started to look for other old buildings and ruins to see if there might be more in my area and quickly found a whole series of sites”.

Survey work began to show that the main centre of the Irish population of barn owls at that time was in the southwest of the island. The overlap with the main distribution of the bank vole was quite noticeable.

O’Clery was hired by BirdWatch Ireland to survey particular areas, to monitor and assess nest sites. It quickly became apparent that one of the best sources of information was from local contacts so he used all available channels in social media, articles in local newspapers, magazines and journals to appeal for information on owls. He put up posters in any shop or garden centre that would take them and the response from the public led to many sightings and new nest and roost sites. Local birdwatchers, though relatively few in number, were alerted to the project and ultimately provided many useful sightings and reports of owls.

Barn owl: The bank vole has helped fuel these birds' expansion in Ireland
Barn owl: The bank vole has helped fuel these birds' expansion in Ireland

Unfortunately, barn owls often become road casualties, especially on motorways where the traffic is fast and they have no chance of escape. The use of GPS transmitters has shown that the birds even hunt preferentially along roadside verges and that they occasionally cross the carriageway at low level, thus coming into contact with traffic. They mainly hunt at dusk and dawn when the poor light means drivers have little chance of avoiding them.

Despite this, the barn owl has expanded, helped by the erection of hundreds of specially designed nesting boxes in trees, farm buildings and on poles. The owls are a welcome addition to a farmyard where they will hunt hundreds of rodents such as rats and mice, reducing the need for rodenticide. The Campaign for Responsible Rodenticide Use in Ireland was established to promote the best practice and responsible use of chemical rodent controls and minimise their exposure to birds of prey, other top predators and pets.

The arrival of the bank vole and another introduced species, the greater white-toothed shrew, has given barn owls more choice of prey, fuelling their expansion. Barn owl groups have been established all over the country to help the birds by installing specially designed nest boxes.

There are now up to 10 pairs breeding in Co Wicklow, so I have placed a nest box in a large tree overlooking a traditional hay meadow on our farm. Each spring I check the box for signs of the owls and I confidently expect that one evening I will see a ghostly white form glide across the meadow hunting mice and voles. What a thrill this would be.

Richard Nairn is an ecologist and writer. His latest book is Future Wild: Nature Restoration in Ireland