Ecology of the tawny owl Strix aluco in the spruce forests of Northumberland and Argyll

Petty, Stephen John (1993). Ecology of the tawny owl Strix aluco in the spruce forests of Northumberland and Argyll. PhD thesis The Open University.



The ecology of the tawny owl was studied in the man-made spruce forests of Kielder, Northumberland (1980-91) and Glenbranter, Argyll (1984-91). Kielder was drier, colder and less mountainous than Glenbranter. Field voles were the main food of tawny owls in both forests. Vole populations exhibited a three-year cycle of abundance in Kielder but showed less variability and were generally less plentiful in Glenbranter. Tawny owls obtained most of their voles from clear-felled areas but used older growth stages in the forest for roosting and nesting. Owl territories were regularly distributed along valleys, with density declining at higher elevations. The numbers of territories increased during the study, probably due to clear-felling which resulted in more food. All aspects of reproduction were significantly related to vole abundance. In Glenbranter compared to Kielder, tawny owls bred at a lower density, territories had a higher occupancy rate with less variation in the proportion of pairs breeding, and on average fewer chicks fledged per pair. In Kielder, most tawny owls failed to breed every third year when vole numbers were low. Without the demands of a family, both male and female owls replaced many more wing feathers than in years when chicks were reared. Population turnover was investigated in Kielder. Here tawny owls showed a high degree of fidelity to their territories and had a high annual survival (84-85%). The median natal dispersal distance was 3.05 km for both males and females, with 30% settling to breed no more than two territories from where they were reared, although none bred in its natal territory. About 80% of chicks recruited into the breeding population were reared in years when vole populations were increasing. There were unoccupied territories in every year studied, and recruitment into the breeding population was the main factor limiting the growth of the population to 4.5% per annum.

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