HOWBARN OWL BATTLES BACK
28 January 2000
HOWBARN OWL BATTLES BACK
The barn owl, one of our most charismatic farmland birds, lost out severely in the first two thirds of the 20th century. However, it now seems that its decline has been pegged back and its numbers have now stabilised at about 4400 breeding pairs in Britain.
Although their decline has stopped, albeit for the time being, changes in the availability of the barn owls preferred prey items could affect its numbers in future.
David Glue, Research Officer for the British Trust for Ornithology (BTO) for over 30 years, has spent many hours pulling apart owl pellets to find what owls eat. Studies of samples gathered over the last 40 years have revealed a disturbing trend.
* Pellet collection
The diet of barn owls in Britain in the early 1900s is illustrated by a unique collection of pellets found in a bricked up farmhouse chimney in Hampshire. While restoring his 16th century farmhouse at Long Candover, Hampshire, David Cooper uncovered the mummified remains of a large number of classically-shaped barn owl pellets sealed in a chimney that had been capped sometime between 1911 and 1913. Although this source represents only one pair of birds, the pellets revealed the presence of an astonishing 14 different types of mammal – far more diverse than any modern barn owl diet so far examined.
* Mammalian prey
The rich mosaic of farmland in the area would have enabled the barn owl to hunt such a large variety of mammalian prey. In periods of bad weather barn owls would have been able to hunt the plentiful mice and rats in the vicinity of the farm buildings, as well as many more frogs, beetles and small birds.
By the 1960s and 1970s a BTO study of pellets from 200 sites of 50,000 prey items showed how the field vole had become the number one prey eaten. Common shrew, wood mouse, bank vole and young brown rats were of lesser importance. This decline in the range of prey taken is probably due to increasing specialisation of agriculture.
In the 1990s, a repeat survey by the mammal society showed how field voles were being taken less frequently. Many smaller mammals, such as pygmy shrew and wood mouse, were being consumed which reduces the energy efficiency of the hunting birds.
The decline in the importance of the field vole was attributed to the loss of grassland while the increase in prominence of the wood mouse is thought to be due to the introduction of set-aside into farmland. In years of local abundance, harvest mice were taken in large numbers.
* Skeletal remains
For David Glue, analysing pellets is never tedious. His fascination for the barn owl has enabled him to build up a detailed picture of the birds diet. "It is amazing just what has turned up. My highlights have included extracting the skeletal remains of stoat, water vole and noctule bat."
David and his researchers have calculated that a single pair of barn owls with an average brood of three or four young, consumed some 3600 to 5000 small mammals each weighing roughly 20g.
Michael Edwards
Barn owls, which will utilise owl boxes as nest and roosting sites, will take harvest mice in great numbers.