DOES NEST-HOLE QUALITY, POOR BREEDING SUCCESS OR FOOD DEPLETION DRIVE THE BREEDING DISPERSAL OF TENGMALM OWLS

1. Four hypotheses have been proposed to explain the breeding dispersal of Tengmalm's owl, a hole-nesting bird of prey which feeds on voles. On the nest-hole quality hypothesis (HQH), the decrease in the breeding frequency in old nest-boxes is due to the deteriorating quality of the box. On the predation risk hypothesis (PRH), the decrease in the use of old boxes is a response to increased predation risk. On the breeding success hypothesis (BSH), the owls disperse more after poor than good reproductive success. On the food depletion hypothesis (FDH), a breeding owl pair may reduce vole density in the vicinity of the nest-box which increases dispersal. 2. The predictions of the four hypotheses were tested using field data collected in western Finland. In autumns 1986-87, 28 nest-boxes were replaced by new boxes at the same place on the tree, 28 boxes were transferred and re-erected within 40 - 100 m from the original tree, 27 boxes were both renewed and relocated, and 79 old boxes were left on the same place on the tree (controls). Breeding frequency in the boxes was recorded during five years after the treatment. 3. HQH predicts that the breeding frequency should increase when an old box is renewed. PRH predicts that the use of old boxes should increase by relocation. However, the renewal and/or the relocation of boxes did not increase subsequent usage. 4. Males were ringed or retrapped at 529 nests (in 1979-91) and females at 655 nests (in 1976-91). A total of 74 males and 31 females were retrapped as breeders one year later. BSH states that dispersal distances should be negatively related to the breeding success in the preceding year, but this was not the case. FDH predicts that the owls should disperse less in the increase phase than in the decrease phase of the vole cycle. The data were consistent with this prediction. 5. The results agreed with the theoretical dispersal models predicting that temporal variation in environmental quality increases dispersal, whereas spatial variation acts in an opposite way.