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# Population Ecology of the Ural Owl Strix uralensis in Central Sweden

Arne Lundberg
Ornis Scandinavica (Scandinavian Journal of Ornithology)
Vol. 12, No. 2 (Aug., 1981), pp. 111-119
DOI: 10.2307/3676035
Stable URL: http://www.jstor.org/stable/3676035
Page Count: 9
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## Abstract

A Ural Owl Strix uralensis population in central Sweden was studied from 1969 to 1978. 84 territories were documented but the true population size probably was close to 100 pairs. Average density was about 0.05 pairs km-2 but in certain areas was as high as $0.25\ \text{pairs}\ {\rm km}^{-2}$. The main prey was small rodents, especially Arvicola terrestris and Microtus agrestis, the former of which made up 60% of the total food biomass. Microtus agrestis and Clethrionomys glareolus varied greatly in numbers between years with peaks in 1969, 1973 and 1977. Vole lows occurred in 1971 and 1975. When voles were scarce the Ural Owl partly fed on other mammals, birds and amphibians. Breeding frequency, clutch-size and production of young varied in synchrony with vole abundance. Mean clutch-size was 2.9 eggs/pair. The annual variation in breeding start was related to vole abundance, and laying was postponed in springs with low vole densities. Clutch-size was strongly correlated to laying date. The clutch on average decreased by $0.12\ {\rm eggs}\ {\rm d}^{-1}$ as the breeding season progressed. Clutch-size was independent of the age of the female. The very low nestling mortality indicates that the most common clutch-size was smaller than the most productive: Mean production of young, including unsuccessful pairs, was 1.12 young/pair. However, more than one third of the population lacked nest-sites and when these pairs were included the annual reproductive output was 0.73 young per pair.

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