Barn Owls, Kestrels & Long-Eared Owls

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Tuesday, 26 June 2012

Invasion Species on the Menu for Ireland's Raptors

Until recently there were five small mammal species on the island of Ireland, that number has now grown by two with the discovery of the Bank Vole and the Greater White-toothed Shrew. The Bank Vole was first discovered in Listowel in County Kerry in 1964, recent DNA evidence has point to the species having been accidentally introduced from Germany in the 1920’s to West Limerick. The Bank Vole has thrived in the Ireland since its introduction and currently occupies a third of the country, while continuing to expand its range. The Greater White-toothed Shrew was discovered more recently, in 2008 while research was being carried out which involved raptor diet analysis, by BirdWatch Ireland, University College Cork and Queen’s University. The species is also expanding its current range out from an area of West Limerick and South Tipperary. Where these two invasive small mammal species occur they can represent a significant proportion of the diet of birds of prey. In some areas of South Tipperary Barn Owls (pictured below my Mick Boldger) have been seen to be feeding almost exclusively on the Greater White-toothed Shrew.

The introduction of these two invasive small mammal species has created distinct small mammal communities across the island. This novel layering up and down of the country, sees three unique regions, one with only native small mammals, another with the native species plus the Bank Vole and the final one which has both invasive species plus our natives. This geographically diversity of assemblages has generated unique ecological effects on both avian predator and on our native Wood Mouse and Pygmy Shrew. The extent of the influence which the Bank Vole and Greater White-toothed Shrew (pictured below by John Murphy) have on raptors seems to be wide-ranging. As of yet little research has been carried out on the Kestrel or the Long-eared Owl. In Barn Owls the Bank Vole can make up to 80% of the diet where they occur in high densities and seem to be having a positive effect on breeding success of the species. The species is also found at higher densities where the Bank Vole is abundant. The situation is more complicated with the Greater White-toothed Shrew, where the species occurs at low densities it has positive effects on Barn Owl productivity and fledging success. However where the species makes up a large part of the Barn Owls diet negative effects have been found. The condition of the chicks, weight and fledging success is reduced in the brood, which can result, in some instances with the entire brood failing. Due to the overlap in ecology of the Barn Owl with both the Kestrel and the Long-eared Owl, similar effects can be anticipated in these two species. It is clear that the present of the Bank Vole and Greater White-toothed Shrew will have a significant influence on the future status of the Kestrel and Long-eared Owl.

Greater-white Toothed Shrew - by John Murphy

The analysis of raptor pellets has pointed to a marked reduction in Pygmy Shrew numbers where the Greater White-toothed Shrew occurs at high densities such as areas of South Tipperary. The theory has been comprehensively verified by Queen’s University small mammal researcher. Their findings demonstrate that the Greater White-toothed Shrew has a positive effect on the Bank Vole, prompting its densities to increase in a given area while affecting local extinction in Pygmy Shrews. The Bank Vole is also seen to gradually replace the Wood Mouse with the effect seen to be reduced the further removed from the point of introduction the community lies.

These two invasive small mammal species are already having top-down and bottom-up effects in terrestrial ecosystems around the country and these will be for the most part negative and will eventually spread across the entire island.    

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