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.    

Research Around the Clock

The morning often sees me trying to track down Kestrel nesting sites and yesterday morning was no different. Between the squalls of a rain laden wind I watched a small shelter belt of conifer and pine which has been a traditional nesting site for Kestrel in the area. The first sign I had of this small falcon’s presence was provided by Rooks. They repeatedly dive bombed, with angry swoops on an area at the centre of the shelter belt. This kind of behaviour is reserved by most birds for predators. It is both a clear statement to the predator that they have been seen and an attempt to drive them away. It is usually performed by a group of birds. Looking for this type of behaviour has become such a habit that the everyday alarm calls^* of a Blackbird or Swallow demand of me, my immediate attention.

Happily the Rooks had done my work for me by finding the Kestrels. There was a pair on site and it wasn’t long before the female returned to her nest. This latest find brings the total of Kestrel pairs found in the West Offaly area to ten, a good number but I am still hopeful that I will find more. Should anyone have information on the location of a nest do be sure to get in touch.

After lunch it’s time to do some Barn Owl, and Kestrel, nest monitoring. These visits aim to record productivity, the relative health of the broods and to ring the chicks. Tuesday, just gone, produced some great numbers. A ruin just outside Banagher played host to six Kestrel chicks and three Barn Owl chicks; all of which were in good health and should all fledge without difficulty. This brood of six represents the largest brood of Kestrels in the country (pictured above). The Barn Owls (pictured below, patiently await us to take their measurements) are quite early, their health and the sheer weight of numbers of Kestrels in the building pay tribute to the rich hunting grounds which the nearby Shannon Callows provides. Another building, close to Tullamore, which has also been a traditional host to both Kestrel and Barn Owls, rounded off the day in some style. We found to our wonder that the Barn Owl nest contained eight eggs. The most BirdWatch Ireland staff have ever seen before in one clutch was six.

The late evening and night of late have seen me carrying out the first phase of a Long-eared Owl chick survey. Monday night saw the completion of phase one. The area surrounding Banagher has proved the most profitable in my search for the calls of young Long-eared Owls (pictured below). I have found three broods within this 10km square but the search will be conducted twice more. It is hoped these later searches will turn up more active sites. Although owls are the marquee nocturnal avian species, they are by no means alone in their nocturnal tendencies. My midnight countryside forays have opened my eyes to the quality of the night time chorus. Grasshopper and Sedge Warblers, Cuckoos, Snipe and Woodcock all broadcast their territorial claims at night. Sadly the afterhour’s cacophonous reel of the Corncrake is now rarely heard in the midlands of Ireland. The summer floods, of a few years past, washed out the nests of this ground nesting bird in the region.

Please do continue to keep in touch with the blog for more avian observations and pictures in the coming weeks as Barn Owls will increasingly be the focus of the project.    

* Interestingly on the subject of alarm calls, there is a bird in Southern Africa called the Fork-tailed Drongo who has learned to mimic the alarm calls of all its neighbours. Drongos watch and wait for a bird to find a tasty morsel, at which point they produce the alarm call of the species in question. The bird instantly flies up in alarm to shelter, leaving behind an easy meal for the Drongo.  

         Barn Owls footage from a Nest Box in south Tipperary

          A big thank you to Brian Dillion for providing the footage and helping out the project

(the footage was collected under licence from the National Parks and Wildlife Service)

This is footage of the female of the pair, who you can see is clearly rung.

                           This is footage of the pair early in the year prior to laying.

This is footage from late April of the first Barn Owl chicks of the season.