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Pierrepont Farm
wildlife monitoring report, June 2013

by Bill Young

Report on owl pellet analysis

Small mammal presence, such as voles, mice and shrews at a site is often determined by humane live trapping using Longworth traps. This method, however, requires three days to complete (one day for pre-baiting with traps open to allow small mammals to become familiarised with the source of food, a second day to re-bait the traps and set them to catch, and a third day to identify and release). There are legal constraints on how long the animals may be kept in the traps before release; 13 hours maximum, less if shrews may be caught. This method would be impracticable to carry out at the farm due to the distance to travel for the surveyor.

An alternative method is to examine the contents of Barn Owl pellets. Barn Owls prey on small mammals, as well as insects, and they swallow the prey whole after killing it. The fur and bones are not digested and are ejected as a pellet, thus facilitating examination. Identification of the bones enables small mammal species to be identified. An estimate of population size would be unreliable by this method, as Barn Owls can range for a radius of about two miles; however, the results would imply that certain small mammals are present.

Barn Owl pellets were collected from the floor of the slatted barn at the farm. There is an owl box in the rafters and a Barn Owl is known to roost there. Two of the pellets were soaked in water to allow the bones to be teased out from the pellets, whilst two were pulled apart dry. The bones were examined under a stereomicroscope, and compared with a Field Studies Council chart of owl pellet contents.


Genus Species Common Name Estimated number Pellet number Method
Clethrionomys glareolus Bank vole 2 1 Dry
Sorex araneus Common shrew 1 Dry
Apodemus sylvaticus Wood mouse 1 2 Dry
Clethrionomys glareolus Bank vole 2 3 Soaked
Clethrionomys glareolus Bank vole 2 4 Soaked
Total 9

From the results above, it may be concluded that these small mammals are present on the Farm, most likely in the long grass field margins. A caveat is that, due to the expected range of the Barn Owl, some may have come from outside of the farm area, although an owl would hunt as near to its roost as possible to conserve energy.

It is recommended that the long grass field margins are retained, to be mown approximately every three years on a rotational basis in order to have suitable habitat always available for small mammals within the owl’s hunting range.

Many thanks are due to Brian Senior and Brian Lavers for assisting with this survey and to Mike and Bev Clear for allowing access.

Bill Young, June 2013.

A less detailed report of the owl pellet analysis (but with photographs) appeared in the blog section of this website in June 2013.