Dalkey Tidy
Brent GoosePainted Lady
Wildlife Newsletter for the Township of Dalkey
December 2009 / January 2010   - Michael Ryan

I have vague memories of seeing a Barn Owl on Dalkey hill many decades ago but I haven’t heard of any in the area in many years. Long Eared Owls aren’t uncommon and in winter time daylight flying Short Eared Owls have been seen on Dalkey Island. Sadly Barn Owls (which used be known as Graveyard Screechers) are in serious decline over most of Ireland but there are some strongholds for them in the West and South West particularly round Kerry and Clare. We found out why at a recent talk given to the South Dublin Branch of Birdwatch Ireland by John Lusby. John is a field worker for Birdwatch Ireland who has been researching the habits of Barn Owls and what might be causing their decline.
In recent years it was thought the Barn Owls decline was due to loss of suitable nesting sites but surveys of nest sites where the owls had nested in previous years showed that they were almost all still intact whereas their former owl occupants had gone. Nest boxes for Barn Owls, which had proven very successful in the UK, had very low occupancy in Ireland.

Barn Owl Photo: Richard T. Mills

Barn Owls diet largely consists of small rodents including pygmy shrews, field mice and rats and it would seem the latter creature could be a major reason for the loss of the Barn Owl. Rats have developed immunity to earlier poisons and newer more effective rodenticides have been developed. Very few of these rat poisons kill instantly, instead relying on continued build up in the rats system as the rat keeps consuming the poisoned bait. This means the rat is still active and moving around over ground while the poison is in its system and it can be caught and eaten by owls, thus transferring the poison to the owl. Barn Owls have very low volumes of stomach acids to break down food and can’t digest the fur and bones of their prey so these are coughed up in the form of cigar shaped pellets. Researchers like John search out these pellets near owl roosting sites then take them for analysis where they can find out what the owls have been eating by identifying the remains of skulls and bones. Doing this resulted in some very interesting findings. In the UK Barn Owls diet has a very high content of Field and Bank Voles, which makes them less susceptible to accumulating rat poison.

Barn Owl Photo: Richard T. Mills

Neither voles are native to Ireland but Bank Voles have been found in the south west of Ireland since the last century and their population is expanding north and east from where they were originally found. There is a fascinating theory that they arrived in machinery that was brought from Germany to help construct the Shannon hydroelectric scheme in the 1920s (see Biology.ie website). Their DNA is a closer match to German bank voles then to ones from our nearer neighbours in Britain. Where the remains of Irish bank voles were found to be the major remains in the pellets of Barn Owls it coincided with areas where there was a good population of the owl.

But an even more fascinating discovery was made during the research work. Bones of a small mammal found in some pellets found in Tipperary and Limerick were proving difficult to identify till the researchers found they were the remains of the Greater White-toothed Shrews, a non native mammal previously unknown in Ireland. Nobody knows how these
creatures arrived in Ireland but they were almost certainly introduced by accident and are now rapidly expanding. John Lusby said, “This is a very significant discovery, which took us all very much by surprise. To find remains of one shrew would have been amazing, but the fact that 53 skulls were found in 10 Barn Owl pellets was phenomenal.” He further commented that “the Bank Vole, which was the last introduced mammal species to be discovered in Ireland back in the mid 1960’s, is now a significant element of the Barn Owl diet where it occurs”. It would seem that the owls that are feeding on introduced mammal species, the bank vole and white toothed shrew, are doing well while barn owls that fed on rats are doing badly. It’s still too early to draw any firm conclusions and there are other factors like the very high fatality rate on roads that have to be taken into consideration but it was very interesting to see the value of long term research into one animal’s lifestyle. Incidentally the animal known as Ratty in The Wind in The Willows is actually a bank vole not a rat. Ironically, they could turn out to be one of the few introduced species that actually does more good then harm