This Update provides more detailed information on several topics covered by the previous version published in 1985 and also develops new research aspects. Loss of habitat and suitable nest-sites, traffic, and poison are main factors causing a decline in most European countries. Food supply has been shown to have a major effect on population size, survival prospects, and breeding biology. Survival also decreases with increasing latitude. Bristish owls differ in several respects from those on the continent. They more frequently breed in trees, hunt during the day, produce fewer second broods (that are also smaller than first broods), their clutch size decreases instead of increases with season, and site/mate fidelity is greater. Owls have been shown to preferentially prey on 10-40 g small mammals. Using telemetry, home-range varies from 90 to 465 ha. Courtship starts in winter and birds can copulate up to 70 times a day. Polyandry is more frequent than polygyni. Feeding rate is on average 3 prey items per nestling per day. Experimental research has been conducted on physiology (energetics), on nestling growth under varying environmental conditions, and on the trade-off faced by parents between current and future reproduction. field experiments showed that siblings negotiate vocally among each other over priority of access to impending food resources, and that degree of spotting of female plumage reflects the offsprings' ability to resist parasites. Finally, in Switzerland, males displaying a reddish brown plumage produced more offspring, and fed their brood at a higher rate than lighter-coloured individuals. Cross fostering experiements were useful to determine the genetics of variation in plumage coloration and spottiness.