Functional response of the Long-eared Owl (Asio otus) to changing prey numbers: a 20-year study

Abstract

The diet of the Long-eared Owl (Asio otus) was investigated in central Slovenia from 1982 to 2001. The Common vole (Microtus arvalis) was the most frequent prey species (55% of all items by number), with a yearly proportion in the diet from 17% to 85%. This frequency of the Common vole reflected its abundance in spring on the meadows, showing a type II functional response - as prey density increased, consumption rate of the owl increased at a decelerating rate, until a plateau was reached. Peak proportions were recorded every 5 to 6 years. The next two most frequent prey species were Wood mouse (Apodemus sylvaticus; 14%) and Field vole (Microtus agrestis; 12%). The Field vole, Common pine vole (Pitymys subterraneus) and Water vole (Arvicola terrestris) were found to be a sporadically important alternative prey. The results are discussed in the light of the optimal foraging theory, which predicts that diet depends on the absolute abundance of the high rank food, that low-ranked prey are dropped from the diet as the abundance of high-ranked prey increases, and that predators do not exhibit "partial preferences", i.e. prey species is either included in the diet or completely excluded. Only the first two predictions were confirmed unequivocally. A method of prey ranking, based on its importance for the predator, using optimal foraging theory predictions, is presented. Accordingly, the Common vole was the only main prey species for the Long-eared Owl. Wood mouse was considered as the only important alternative prey, and Field vole, Common pine vole and Water vole as sporadically important alternative prey species. All others were alternative prey of low importance.