Research was carried out in the years 1982-1984. Total 259 pellets of Strix aluco were collected from 26 stations (12 territories). Among prey 767 individuals of vertebrates and 140 invertebrates were distinguished (Table 1 and 2). The presence of 16 mammal species belonging to two orders: Rodentia and Insesctivora (96.2% of the total prey biomass) and 3 bird species from Passeriformes, was found. Rodents dominated by weight, and among them the Arvicolidae family. Among invertebrates representatives of 7 Insecta orders and of snails, were found. Coleoptern, mainly large Carabidae species were dominants (96%) among Insecta. The food studied was divided into 3 categories, by weight of individual prey in the total prey biomass: 1) Basic - (prey above 5% by weight): Microtus arvalis (39.3%), Clethriomys glareolus (15.3%), Apodemus sylvaticus (7.1%), Pitymus subterraneus (5.3%) and Arvicola terrestris (5.8%). The basic food constituted 80.4% of biomass being Tawny Owls’s diet. 2) Complementary - (prey 1-5% by weight): Talpa europaea (2.4%), Apodemus flavicollis (2.2%), Microtus agrestis (2.2%), Sorex araneus (1.4%), Mus musculus (1.2%) and birds (3.8%). Complementary food constituted 18.0% of the diet biomass. 3) Occassional - (prey below 1% by weight): Apodemus agrarius (0.6%), Micromys minutus (0.4%), Apodemus microps (0.3%), Neomys fodiens (0.2%), Sorex minutus (0.1%), Crocidura suaveolens (0.04%). This group also comprised insects (approx. 0.1%). In comparison to results of other studies a high percentage of rodents was found, and no represantatives of Pisces, Amphibia, Reptilia, Mustellidae, Gliridae, Lagomorpha and Chiroptera. This could be explained by: the small amount of analysed material, by the fact that it was collected between October and March and by the different types of forest studied by particular authors. On two plots studied, differing by the percentage of forest clearings a different composition of Strix aluco diet was found (Table 3). High frequency by biomass of Microtus arvalis (48.7%) was found in the plot "forest with clearings" (29% of clearings). In the plot "dense forest" (8% of clearings) the portion of this prey was twice as low (24.0%), and other mammals, such as Apodemus flavicollis, Sorex araneus and Arvicola terrestris and birds, were substitutes.
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