Population Trends of North Finnish Land Birds in Relation to Their Habitat Selection and Changes in Forest Structure

Abstract

Long-term trends in the 22 most abundant land bird species breeding in northern Finland were related to their habitat selection patterns. Independent data sets were used to describe long-term population trends (POP), edge preferences (EDGE) and the effects of the changing age structure of the forests on bird density (AGE). AGE combined the present age preferences of birds with data on the changing age structure of North Finnish forests from the 1950s to the 1970s. EDGE and AGE correlated positively though not significantly with POP. When EDGE and AGE were taken into account simultaneously, a significant correlation with POP emerged (67% of the variance accounted). Southern species were an exception; their population trends seem to depend on changes in southern Finland. The sedentary species of old forests have plummeted during the recent decades in northern Finland. The results for this group (Parus montanus, P. cristatus, P. cinctus, Certhia familiaris, Perisoreus infaustus) agreed well with the finding that fragmentation and changes in the age structure of the forests are mainly responsible for the recent trends among the abundant forest birds in the north. Our results seemingly indicate a close tracking of the environmental resources by the bird community. However, as the geographical scale of the study is broad (northern Finland as opposed to one population site) and the temporal scale decades rather than years, a simple saturation hypothesis is not an inevitable inference from the data; and if there is close tracking of the environmental resources, it is rather in terms of populations and not of the whole community. An alternative hypothesis is provided by changing numbers of high-quality and low-quality population sites; in this view, the regional pattern is a result of complicated dynamics in a mosaic of local populations.