Climatic Changes, Habitat Changes, and Competition: Dynamics of Geographical Overlap in Two Pairs of Congeneric Bird Species in Finland

Abstract

The relative densities and overlap zones of Parus cinctus (northern) and P. cristatus (southern), and of Fringilla montifringilla (northern) and F. coelebs (southern) were studied in censuses made in Finland in 1910-29, 1936-49, 1952-63 and 1973-77. The population changes of the species are described. P. cinctus greatly decreased, first in the southern and later in the middle parts of its range. P. cristatus first expanded both northwards and to the Aland Islands, but then decreased, particularly in the north. In their overlap zone, both species decreased drastically (by 90-95%). F. montifringilla was stable, except for a temporary decrease in the 1940's. F. coelebs increased, but the 1:1 zone of the Fringilla species has fluctuated mainly owing to fluctuations in the numbers and range of F. montifringilla. Two major theories purport to account for the dynamics of North European biota: either the climatic amelioration of the past 100 yr is of major importance, or recent habitat changes (often due to man or his livestock) are more effective. Recent changes in climate and habitats are compared with bird population changes in order to test the two biogeographic theories. Climatic amelioration may account for some, but certainly not all population changes. Habitat changes (including the abandonment of forest grazing, the increase of spruce, increased edge effect, and extensive clear-cuttings particularly in N Finland) may explain all long-term trends. A few short-term population changes were probably due to exceptional winter or spring weather. The presumed interspecific competition between the congeners studied is examined critically. Standard competition equations do not adequately model the dynamics of the species pairs, but at least three additional points must be taken into account: environmental changes, population changes in relation to ecological saturation, and the evolutionary time scale.