Nest Hole Shift in Tengmalm's Owl Aegolius funereus as Defence Against Nest Predation Involving Long-Term Memory in the Predator

(1) Nest predation in Tengmalm's owls using nest boxes was studied for 13 years in southeastern Norway. Forty-eight per cent of the clutches were preyed upon, and of these at least seventy per cent, and possibly all, were taken by pine martens. Nest checking by humans did not influence the frequency of predation. (2) Frequency of predation increased with nest box age, but was independent of nest density, which fluctuated in parallel with numbers of small rodents, the staple prey of Tengmalm's owls and pine martens. Tengmalm's owl clutches had a higher probability of being preyed upon in nest boxes where the previous clutch had been taken than in boxes where there had been a successful nesting the previous year. These results indicate that pine martens did not encounter nest boxes randomly, but revisited those that they had found previously. (3) Tengmalm's owls preferred to nest in new boxes, and a majority of the females shifted nest hole between successful nestings. Clutches initiated in March were in newer boxes than were later clutches, and were thus less exposed to predation. (4) Predators which revisit the nest holes they have found will put a high risk on clutches laid in these holes. In hole nesting birds exposed to such predators, this will select for individuals which move to another hole after having a clutch preyed upon. If new holes become available each year it will also select for individuals which shift nest hole between successful nestings and prefer nesting in new holes.