Spatial and temporal changes in the diet composition of the Eurasian eagle-owl (Bubo bubo) in Slovakia comparing three historical periods

Abstract The author evaluates his own data on the food of the Eurasian eagle-owl (Bubo bubo) in Slovakia using material he collected between 1975 and 2020. A total of 105,543 food items were identified in 254 samples taken at 136 localities. Mammals had the highest representation (Mammalia, 65 species, 58.4%), and the species composition of birds was diverse (Aves, minimally 140 species, 8.5%), but the common frog (Rana temporaria, 32.0%), from the lower vertebrates, is represented more abundantly. Invertebrates (Evertebrata, 0.1%) occurred in food residues only occasionally. The bulk of the samples were collected from eagle-owl nests. The samples were divided into three time periods (A–C), which differ in the manner of human land-use management: A up to the 1950s, with a smaller area of field plots and more extensive grazing in the uplands; B from the 1950s to the 1980s, during the Socialist period, with the concentration of agricultural production in large units; C the last 30 years, 1990 to 2020, with the gradual break-up of collective land management. The first period (A) is characterised by a strong dominance of frogs, particularly the European brown frog R. temporaria (44.6%), and a large share of small mammal species of the family Muridae (genera Apodemus and Mus). During the time of Socialism (B), eagle-owls adapted to hunting larger species of mammals and birds, and the share of frogs in their food fell by half (R. temporaria, 23.3%). With the decline in livestock production after 1990 (period C), the species diversity of birds increased: aquatic species and raptors in particular are on the rise. Successive overgrowth of pastures in the submontane zone is reducing the hunting territories of eagle-owls. The dominance of the common vole (Microtus arvalis) in their diet has gradually increased from period A (26.8%) to period C (37.3%). Data from eleven areas around Slovakia are evaluated separately for the three time periods. In period A, the highest proportion of frogs was in the Liptov region (R. temporaria, 68.2%), when eagle-owls nested deeper in the mountains. The proportion of frogs decreased towards lower areas, and in the Ponitrie (Nitra river basin) it was only 10.8%. At the same time, the share of M. arvalis and larger prey increased. A similar trend of increasing shares of larger prey towards lower locations also applied during the Socialist period (B). In the last 30 years (C), frogs in the higher river basins have given way to European water voles Arvicola amphibius and M. arvalis. In association with the progressive overgrowth of pastures, forest species such as the yellow-necked mouse (Apodemus flavicollis) and bank vole (Myodes glareolus) are increasingly prevalent, as are the white-breasted hedgehog (Erinaceus roumanicus) and various thrushes (Turdus sp.).