Density, diet and productivity of Long-eared Owls Asio otus in the Italian Alps: The importance of Microtus voles

Capsule: Relatively large populations, feeding predominantly upon voles, were present at higher elevations. Aims: To determine the density, productivity and diet composition of Long-eared Owls breeding at higher elevations. Methods: Population census and breeding biology were investigated, and dietary analysis performed for 32 Long-eared Owl territories that were occupied over a six-year period (2000-05), in a 155 km2 study area located in the Noce Valley of the central-eastern Italian Alps. Results: Territories were mainly situated at the edge of large pine forests, near large patches of grassland and/or open-structured apple orchards, at elevations ranging between 540 and 1210 m. Density varied between 10 and 15 pairs/100 km2. Mean intraspecific nest spacing averaged 1727 m and territories were either solitary or clumped in loose aggregations of one to five pairs. Mean laying date was 27 March and the mean number of fledged young was 0.95 and 2.13 per territorial and successful pair, respectively. Adults and nestlings were preyed upon by Eagle Owls Bubo bubo and Common Buzzards Buteo buteo, both of them abundant in the study area. Diet was dominated by Common Voles Microtus arvalis, complemented by Apodemus mice and thrushes. Annual variations in owl density, productivity and diet breadth varied in parallel with the occurrence of Common Voles in the diet, suggesting that the owls responded numerically to the availability of their main prey. This confirms earlier analyses on other European populations, but without the marked three- to four-year cycles observed in such areas. Conclusion: The conservation status of the species in these Alpine habitats seemed currently satisfactory. However, the unregulated use of rodenticides in apple orchards and the loss of open habitats associated with land abandonment may represent long-term threats for the species in these habitats. © 2008 British Trust for Ornithology.Peer Reviewe