I was doing some reading on this species in Handbook of Birds of the World Vol 5 (1999) which makes the comment that population estimates are largely conjectural, even in Europe, owing to species' elusive nature and mostly nocturnal habits, and primary association with relatively inaccessible areas of coniferous forest. European strongholds in Sweden (10,000-40,000 pairs) and Finland (8000-20,000 pairs). Russian population estimated at 10,000-100,000 pairs. Approximately 11,000 nestboxes have been installed in Finland from which fledglings are ringed annuallly. Fluctuations in breeding populations appear dependent on vole cycles and snow depth. Indirect effects of forest-cutting probably greatest human influence with reducing prey populations, removing forest structure necessary for foraging and eliminating nest cavities. Selective harvesting may allow tree removal while maintaining suitable habitat.