Estimating population dynamics and dispersal distances of owls from nationally coordinated ringing data in Finland

Abstract

Amateur bird ringers can collect data at a geographic and temporal scale that is rarely possible with professional field crews, thus allowing truly national analyses of population dynamics and dispersal. Since the early 1970s, bird ringers in Finland have been strongly encouraged to focus on birds of prey, especially cavity–nesting owls. In addition to ringing nestlings and adults, ringers also provide data on population trends and breeding success. The resultant data indicate that numbers of breeding pairs fluctuated with the 3–4 year microtine cycle, but without any long–term trend. Mean productivity per nest varied from 2.18 to 3.33 fledglings per active nest in Tawny Owls, 1.56 to 2.87 in Ural Owls and 1.78 to 4.32 in Tengmalm’s Owls. Survival and breeding propensity also varied with the vole cycle and explained much of the observed variation in breeding populations. Observed median dispersal distances were 24 and 18 km for Ural and Tawny Owls respectively, but increased to 36 and 48 km, using a method presented here to adjust for uneven sampling effort, highlighting the importance of considering sampling effort when estimating dispersal.