Tawny Owl (Strix aluco) Predation in a Pristine Deciduous Forest (Bialowieza National Park, Poland)

Abstract

1. Tawny owl Strix aluco predation on its main prey (bank voles Clethrionomys glareolus and yellow-necked mice Apodemus flavicollis) and alternative prey (shrews, birds, and amphibians) was studied in the pristine deciduous forests of eastern Poland during a 7-year period (1985/86-1991/92) that included 5 years of moderate densities of rodents and 2 years of outbreak and crash triggered by a heavy seed crop of oak, hornbeam, and maple. 2. Number of resident owls was rather stable; 45-53 owls per 10km 2 in years of moderate and high rodent numbers, and 33 owls per 10km 2 in a rodent crash year. Numerical response of owls to rodent densities was log shaped. 3. In the cold season, the dietary (functional) response of owls to autumn density of rodents was of type II (logarithmic). No functional response of owls was observed to shrew numbers. Owl hunting rate on amphibians in winter was shaped by the mean temperature of the cold season and, to a lesser extent, by availability of rodents. Owl hunting rate on birds was inversely related to rodent abundance. 4. During the seven cold seasons (1 October-15 April), tawny owls removed 3-19 mice, 2-18 voles, 2-7 shrews, 0-2 birds, and 1-9 frogs from an average hectare of the forest. Total predation on rodents (N prey removed ha -1 ) was lowest during the crash of rodents and highest during the outbreak; it grew logarithmically with increasing density of rodents. On average, in the cold season, tawny owls removed 54% of autumn numbers of mice and 40% of those of bank voles. Percentage predation peaked at 22 rodents ha -1 in autumn. At both higher and lower prey densities, percentage predation declined. 5. The percentage predation curve of tawny owl was compared to the frequency distribution of autumn densities of rodents in Bialowieza (from 23-year trapping). The heaviest predation fell to prey densities that occurred most frequently. 6. In summer seasons (July-September 1988-92), tawny owls (both adults and juveniles) consumed 0.5-19 mice and 0.4-10 voles from an average hectare. Percentage predation varied from 6% to 74% of rodent numbers recorded in July, but rodents bred rapidly and recruitment usually more than compensated for losses due to predation. 7. Total predation by tawny owls on rodents was quite stable as a consequence of high stability of owl numbers. Percentage predation, although heavy in all years except for rodent outbreak, was not density-dependent and therefore tawny owls did not regulate rodent numbers. In the deciduous forests of Bialowieza National Park, the pattern of predation by tawny owl, a resident generalist, did not differ from that by the weasel, a resident specialist.