Analysing owl pellets from Skomer Island

As part of our third year Avian Biology module, students analysed owl pellets from Skomer Island, South Wales. The pellets had been kindly collected in Autumn 2015
by the island wardens and given to Matt Wood.
In most cases, these were from a barn owl Tyto alba, apparently roosting in a disused chimney, which gave the pellets (and the lab!) a rather pleasant smoky smell. In common with many birds of prey, owls often consume whole prey items such as mice, voles and shrews and then regurgitate the indigestible parts of their meal, including hair and bones. By soaking a pellet and carefully pulling it apart, it is possible to identify the prey. The different mammal species are best identified by their lower jaw bones. Although small mammals are most common, it is possible to find birds, bats, amphibians and insect remains too. 
  Of the 15 barn owl pellets analysed, 14 contained field vole Microtus agrestis exclusively. In most cases this would not be unusual as it is one of the owls’ favourite foods. In this case, though, it was quite a surprising result… because there are no field voles on Skomer! This is good evidence that the barn owls were roosting on Skomer and foraging on the mainland, perhaps with only opportunistic hunting on the island itself. This agrees with previous analysis of Short-Eared Owls Asio flammeus by Skomer’s wardens (http://skomerisland.blogspot.co.uk/2015/03/winter-leaves-its-mark.html) and also previous dissertation projects by University of Gloucestershire students Emily Compton and Elle Daley on Short-Eared and Little Owls Athene noctua (stay tuned for their paper… in review!): Skomer’s owls range widely to forage off the island. Of the barn owl pellets, only one contained a mammal species actually resident on Skomer: a wood mouse Apodemus sylvaticus.

 
The 16th pellet was from a Short-Eared Owl, and held perhaps the biggest surprise seabird remains. Based on the size of the bone, the shape of the leg bones, and the feathers in the pellet, this is likely to be one of the many Manx shearwaters Puffinus puffinus found on Skomer, and more evidence of opportunistic hunting.  

This just shows the value of owl pellets: fascinating insights can be made into science and natural history questions, all within a module-based lab practical!

Anne Goodenough, Reader in Applied Ecology

Matt Wood, Senior Lecturer in Biosciences

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