The use of barn owl Tyto alba pellets to assess population change in small mammals

Capsule: Barn Owl pellets can be used to assess population changes in small mammals though with certain important reservations. Aims: To conduct a large-scale, long-term analysis of pellets regurgitated by Barn Owls collected from a defined area of countryside within the UK south Midlands and compare results with those of other studies that have used pellet analysis to investigate the diet of Barn Owls. Methods: Over 61 000 Barn Owl pellets collected over 20 years in the south Midlands of England were analyzed. Results: We recorded a steady increase in the proportion of Field Voles in pellets over time at the expense of Wood Mice, and a positive relationship between various rainfall measures and the proportion of Field Voles in pellets.We suggest that the conclusions of previous studies of the diet of Barn Owls have often been predicated on inconsistent or untenable assumptions. Conclusion: We challenge some of the premises on which Barn Owl pellet analyses have traditionally been based.We do not think that either a shortage of Field Voles in pellets, or of pellets containing the remains of more, smaller prey items, are indicative of foraging difficulty. Barn Owl pellets can be used to assess population changes in small mammals, so long as possible causes of bias, which we discuss, are acknowledged.