Clutch size and breeding success in relation to nest‐box size in Tengmalm's owl Aegolius funereus

Abstract

Clutch size and breeding success was related to nest‐box size in 215 nests of Tengmalm's owl in the Kauhava region, western Finland in 1966–82. In vole peak years, but not in other years, mean clutch size was positively correlated with the bottom area of the nest‐box, increasing on average by 0.005 eggs cm−2. In successful nests there were fewer nestlings in small and medium‐sized than in large boxes. Breeding was most successful in thick‐walled boxes with a small entrance diameter. The optimal box for Tengmalm's owl has the following dimensions: bottom area 20 × 20 cm, entrance diameter 8 cm, and wall thickness at least 3 cm.Hypotheses suggested to explain the “area effect” based on breeding data of hole‐nesting passerines do not hold good for Tengmalm's owl. Since there were more stored prey animals in medium‐sized and large boxes than in small boxes, I suggest that the “area effect” is related to food availability, which may determine how sensitive the species is to different bottom areas of the boxes, at least in areas with cyclic food (e.g. northern Europe).