Häckningsframgång hos ormvråk Buteo buteo och kattuggla Strix aluco i Varbergs kommun 1984–2016

In 1984–2016 we studied the habitat choice, nest placement and breeding success of Common Buzzard Buteo buteo and Tawny Owl Strix aluco in the municipality of Varberg in southwest Sweden (57°10'N, 12°10'E). The analyses are based on 1,512 successful breeding attempts of Common Buzzard, and 1,387 of Tawny Owl. The average number of young per successful breeding was 1.74 and 2.86, respectively. The variation between years was highly correlated between the two species, suggesting a common food source. In both species the between-year variation in clutch size was particularly large in beech forest territories, most likely caused by the effect of beech mast years on local rodent populations. The breeding density of Common Buzzard was particularly high in 1990–1995, coinciding with large areas of farmland set-aside. In the Common Buzzard, the yearly average number of young per clutch declined significantly with yearly average hatching date.