Diet of tawny owls (Strix aluco) in relation to field vole (Microtus agrestis) abundance in a conifer forest in northern England

The diet of tawny owls Strix aluco was determined from pellets and prey ite ms in owl nests in Kielder Forest, a planted spruce forest in northern Engl and. Field voles Microtus agrestis were their most important food, and form ed the highest proportion of tawny owl diet in winter and early spring. Com mon shrews Solex araneus, common frogs Rana temporaria and birds were taken more frequently in late spring and summer. Clear cuts, areas from which ti mber had been felled at the end of the rotation, provided the main field vo le habitat in the forest and remained suitable for voles for 10-15 years af ter re-planting. Field vole abundance was measured three times a year on nu merous clear cuts throughout the study area using a vole sign index based o n fresh grass clippings in runways. Tawny owls responded functionally to th e 3 to 4-year cycles of field vole abundance. In years when voles were scar ce, adult owls took more common shrews and common frogs, as determined from pellet analysis. In contrast, more bird prey was fed to nestlings when fie ld voles were scarce, as determined from prey items in nests. The proportio ns of the main prey in nests changed over a 19-year period. More bank voles Clethlionomys glaveolus and wood mice Apodemus sylvaticus occurred in ever y year after 1992 than in any year before this. Numbers of wood mice in owl nests increased significantly throughout the study period, whereas bank vo le numbers exhibited noncyclic, multi-annual fluctuations that were unrelat ed to field vole cycles. It is argued that fluctuations in rodent prey refl ected changes in rodent guilds in the study area; reasons for this are disc ussed. This is the first study of tawny owl diet in spruce forests in Brita in and highlights the value of such large-scale dynamic habitats for rodent populations and their predators.