Clutch size and breeding success of Tengmalm’s Owl golius funereus in natural cavities and nest boxes.

Altogether 287 nests of Tengmalm s Owl golius funereus were visited in 1960-82 in westem Finland. The-clurches were si’nificantly larger in nest-boxes than in natural cavities. The hatching percentage and number of fledglings also seemed to be higher in boxes than in natural nesting places. But the drifference was not significant. Clutch size in Tengmalm s Owls positively correlated with the bottom area of the box and this ‘area effect is responsible for the smaller clutches in natural holes. Poor breedins success in natural holes may be due to poor insulation of the cavity or to ice, water or old nest material at the bottom of the nest. lt may also be caused by the hyperthermia occurring among large broods on hot days, increased cannibalism in small holes and a high degree of predation in cavities with large entrances. As the numbers of fledglings are generally smaller in natural cavities than in nest-boxes, studies of box populations may give misleadingly large breeding results for hole-nesting birds

Korpimaki, E. 1984, Ornis Fennica 61:80-83