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Wednesday, 31 July 2019

Invasive Shrew has big effect on Barn Owls

Just two weeks ago, remains of Greater White-toothed Shrews were found in two Barn Owl pellets at a site just a few hundred metres into Co. Kerry, so this fast-invading small mammal is either already in Kerry, or right on the border. Although it had been expected to reach Kerry soon, there have been several 'range jumps' elsewhere this year with evidence from John Lusby that they are now in NE Galway and even Mayo (see this post HERE for more detail on its recent spread.

One of three Barn Owl chicks, just about fledged, at a nest site in a castle in east Kerry (All nest visits and ringing carried out under licence, NPWS. Photo: Michael O'Clery).

As Barn Owl ringing has been underway in the country over the past month it has been increasingly clear that the range of the Shrew is having a big impact on the local breeding success of Barn Owls. In the core range of the Shrew in Tipperary, Limerick and north Cork, Barn Owls have been gorging themselves on them, it often forming 80% or more of individual owls diets. As a result, the well-fed owls are having a bumper year with egg-laying an average of three weeks earlier than normal, and with larger average brood sizes. Two broods of six Barn Owls have been found at two nests in Tipperary, only the third and fourth time this has been recorded in Ireland (the first was in Kerry in 2015, see this post HERE). New sites have also sprung up in this and nearby areas, ie, Kilkenny, and this summer, Barn Owls in at least three sites in the core range of the Shrew are currently attempting to have second broods - the first time this has been recorded in the wild in Ireland.

 Two chicks from a brood of three, Co. Kerry, July 2029 (Michael O'Clery).

In Co. Kerry, where the Shrew has not yet had any real impact on Barn Owls, the picture hasn't been quite so good this summer. As elsewhere in the country, Barn Owls in Kerry have nested on average three weeks earlier than usual, but unfortunately site occupancy has been relatively low and brood sizes about average. So while not a bumper year for Kerry Barn Owls, it's certainly not the worst year they have had.

Some Co. Kerry Barn Owl sites, such as the one above, remain strangely unoccupied. This site has had nesting Barn Owls for over thirty years, but they have not been present for the last two summers despite the nest site remaining suitable and the area surrounded by ideal foraging habitat (Michael O'Clery).

One strange phenomena this summer has been a strong preponderance of male chicks. In Kerry, where the sex could be accurately determined, 11 of 14 chicks ringed were male. It's not known why this might be the case.

A fledged male Barn Owl at a tree nest site near Tralee, July 2019 (Michael O'Clery).

 Although most owls have now already fledged, because of the early egg-laying dates we will continue to monitor nest sites into late summer and early autumn as it's possible that there could still be a second brood in Kerry this year. Let's hope so. 

There seems little doubt that the Greater White-toothed Shrew will have a positive effect on Barn Owls in Kerry in the coming years. Not so great if you are a Pygmy Shrew or Wood Mouse, as both go into serious decline wherever the Shrew invades, but the effect on our Barn Owl population will likely be for the better. It is also likely that Kestrels and Long-eared Owls, and possibly Hen Harriers, are benefiting too. For better and for worse, the Shrew is about to become a permanent and abundant addition to the county's fauna and nothing will stop it now.