About the Barn Owl
Barn Owl pellets, diet, and predator-prey relationships
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What is a Barn Owl Pellet?
Barn Owls generally swallow their prey whole but are unable to digest the hair and bone. After each night's hunting the owl regurgitates one or two black pellets typically about the size of a man's thumb and containing the remains of four or five small mammals. In dry well-used roost sites lots of Barn Owl pellets can accumulate and using the Trust's age determination guide it's now possible to work out how recently the owl was present...
Many thousands of owl pellets have been analysed so a great deal of information on Barn Owl diet is available. In the UK the most frequently taken prey is the field vole, Microtus agrestis (also known as the short-tailed vole) usually forming between 40% and 80% of the diet. In most diets, the long-tailed field mouse, Apodemus sylvaticus (also known as the wood mouse) or common shrew, Sorex araneus is the second most frequently taken. Although not often important by number, the brown rat, Rattus norvegicus is important when the body weights of the various prey are considered.
Other species of vole, mouse, and shrew are taken, and in certain diets or at particular times they can be important. In the UK, amphibians, reptiles, insects, birds, bats, rabbits and carrion are not generally taken by Barn Owls and earth worms are not eaten at all. However, unusual cases have been recorded where individual Barn Owls have taken considerable numbers of frogs or small birds in winter. Cannabalism in Barn Owl nests has been recorded but is unusual.
In the course of a year, a breeding pair of Barn Owls needs roughly 4,000 prey items (4/night x 2 adults x 365 days + 4/night x average brood of 3.5 x 70 days) and there are lots of other predators that eat small mammals too. Because so many small mammals are being caught all the time you may think that small mammal populations are controlled by the amount of predation they suffer. In fact, the reverse is true. The predators are controlled by the prey!
By far the most thorough long-term study of Barn Owls in Britain was carried out by Dr Iain Taylor over an 11-year period in SW Scotland and Iain's book "Barn Owls, Predator-Prey Relationships and Conservation" was required reading for all Barn Owl Trust conservation staff until we produced the Barn Owl Conservation Handbook in 2012. Taylor demonstrated that food availability varied a great deal year on year and controlled the size of the Barn Owl population in SW Scotland far more than any other factor. Field vole density can vary from 15 to 400 individuals per hectare. In years when field voles were numerous the Barn Owls nested earlier, laid more eggs, and produced more young. The survival of young and adults was also largely controlled by prey availability.
In SW England where the Barn Owl Trust is based, small mammal numbers show less annual variation than further north and there are other powerful influences on owl survival. Even so, prey availability is still the most important factor in determining the size of the owl population.
There is more information about this topic in the Barn Owl Conservation Handbook.