Experimental increase of predation risk induces breeding dispersal of Tengmalm's owl

Nest predation and its avoidance are critical components of an individual's fitness and play an important role in life history evolution. Almost all s tudies on this topic have been observational, and thus have not been able t o separate the effects of individual quality, habitat selection and predati on risk of given nest sites from each other. More experimental studies on n est predation and breeding dispersal, therefore, are needed to avoid confus ing interpretations of the results. In western Finland, pine mar ten (Marte s martes) predation risk was experimentally simulated at the nests of Tengm alm's owls (Aegolius funereus) by using a caged American mink (Mustela viso n) as a predator. Nests without exposure to a mink served as controls. In a ccordance with our predictions and earlier observational studies, males exp osed to simulated predation risk increased nest-hole shift and breeding dis persal distances compared to control males. Nest-hole shift and long breedi ng dispersal distances probably decrease the risk of nest predation, becaus e pine martens are known to revisit nest-holes they have found.