Breeding of the tawny owl Strix aluco in Finland: responses of a southern colonist to the highly variable environment of the north

Abstract

Large-scale patterns and variations in the occurrence and breeding of the Tawny Owl Strix aluco in Finland was studied in relation to the availability of small voles and winter-ing birds as well as to general weather conditions of the preceding winter. In 19862000, pronounced three-year cycles in the numbers of occupied territories as well as nesting at-tempts recorded were less regular near the southern coast than elsewhere in Finland. On average, 64.7 ± 6.8 (SD)% of the Tawny Owl territories recorded were annually found to be occupied by breeding birds. The annual mean of the clutch size averaged 3.67 ± 0.46 (SD), and that of the fledglings produced per nesting attempt 2.73 ± 0.37 (SD). The annual mean fledgling production per occupied territory was significantly lower in the southern coastal region than elsewhere in Finland. The average long-term fledgling production of Finnish Tawny Owls was high, but not consistently higher compared to that of more southern populations. The clutch size and fledgling production followed mainly the cycli-cally fluctuating levels of small voles. High numbers of wintering birds seemed to have favourable effects on breeding, while benign weather conditions of the preceding winter tended to increase the number and proportion of breeding birds.