A nest-box breeding population of Pied Flycatcher Ficedula hypoleuca in subalpine birch forest at Ammarnäs in northern Sweden was studied in 1965–2010. The population showed on average a significantly decreasing trend during the initial 25 years, after which it remained on a static level. The yearly variation of the population size was significantly correlated with breeding result in preceding years, which in turn was affected by e.g. nest predation, adverse climatic factors, clutch size, and incidence of defective egg shell formation. It is also suggested that the population decline during the initial 10–15 study years followed from over-establishment of the breeding habitat. The average breeding result was 2.3 fledglings per pair, which is far below the number needed for the population to be self-reproducing. The occurrence of unusually high rates of nest predation and defective egg shells seems to be passing phenomena, predicted to approach zero after about 30 and 60 years, respectively, after the start of the study. But even with these factors eliminated and with other conditions remaining unchanged, the Ammarnäs population will continue to be a sink population.