Egg-laying is between normally between late March and early May in most of the range. Egg laying dates in the northeastern United States, in a sample of 42 eggs, were between March 14 and May 30. Additional records in southern Canada show laying as occurring at as late as June 5. 43 eggs in Ontario were laid between March 19 to May 24, more than half between April 15 and May 5. In northern Italy, the mean egg laying date was March 27. Exceptional eggs were recorded as early as December 31 to January 3 in Spain. Re-nesting can occur within about 20 days after a clutch is lost. In Europe up to two broods have been successfully been raised at times of food abundance. Generally the female lays 3-5 eggs (sometimes more if food is unusually abundant). Larger clutches are typical further north in the range. Record sized clutches have been recorded for Europe of up to 8 eggs in Sweden in years of a “vole plague” while record sized clutches worldwide were recorded of 10–11 in the Kazan area of Russia in similar conditions. In Great Britain, the average clutch size was recorded as 3.9 while in Germany it was 5.5 and, similarly, in Slovenia it was 5.6. In a study from Montana, the mean clutch size was 5. The pure white eggs are on average in both North America and central Europe and weight about . At 1-5 day intervals (on average 2), the eggs are laid on the bottom of nesting area. A clutch of 7 eggs takes 10 to 11 days to lay. Incubation begins with the first egg and continues for about 27–28 days (in extreme cases from about 21 to 30 days). The female alone incubates while the male provides food, which is brought direct to nest. She may leave the nest early on to feed but does so much less later into incubation. A study in Montana found that corticosterone levels were considerably higher in adults of both sexes during the breeding season than during the non-breeding season, suggesting that the breeding season is more stressful on the owls.