SELECTION FOR NEST-HOLE SHIFT AND TACTICS OF BREEDING DISPERSAL IN TENGMALM OWL AEGOLIUS-FUNEREUS

(1) Factors selecting for nest-hole shift and the success of two tactics of breeding dispersal in Tengmalm''s owl were studied for 20 years in western Finland using nesting boxes. (2) Mainly because of nest-site scarcity and the cyclic food source (voles), most adult males were resident whereas most females were nomadic. The males shifted the nest-hole only within their own territories, but the females often changed territory. The frequency of movements correlated negatively with the fluctuations in microtines. (3) Food supply was the main reason for nest-hole shift. The males changed nest-holes within their territories in order to avoid areas that were depleted of food, and the degree of nomadism in the females was also mainly regulated by food. (4) Five per cent of the clutches were destroyed by pine martens. The frequency of predation was independent of hole type, nest-box age, nest density and success of the previous clutch in the box. Pine martens did not revisit holes and boxes found previously when hunting, since alternative prey to owl clutches was much more abundant. (5) Boxes made of boards were preferred to those hollowed out of logs, but tree holes were used as frequently as board boxes. The owls chose the larger holes, especially in good vole years, when large clutches needed plenty of room. Avoidance of predation did not affect their choice. (6) immigrants performed less well in individual breeding attempts than did the residents. This may be due to unfamiliarity of the former with their territories and/or to owls tending to stay only on good territories.