Abstract
A long-term Boreal Owl Aegolius funereus nest box project ran 1976–1985 in central Sweden. Average occupancy among 200 nest boxes was 16% (range 0–54%) with average clutch size 6.0 eggs and number of fledged per successful breeding 4.6, while 41% of breeding attempts failed, primarily due to predation by pine marten Martes martes or desertion by the female. The highest observed breeding density was 0.9 breedings/km2. Five cases of bigamy were observed. Females weighed 50% more than males during egg laying and incubation, probably because of pre-emptive weight gain to compensate for the risk of subsequent changes to food availability. The male provisioned both the female and chicks for most of the breeding period, mainly with small rodents (87%). Population sizes of small rodents fluctuated in 3–4-year cycles. Reproduction was successful during phases of increasing rodent density (1977–1978, 1980–1981, and 1984–1985), while almost no breedings were initiated during nadir years (1976, 1979, and 1982–1983). Ringing recoveries suggested that juveniles recruited into the local population in years with abundant food resources, but dispersed if food availability was declining. Adult females were often nomadic, while most males were resident.