Some Climatic Limits in the Extension of Birds in Eastern Europe

Abstract

Inasmuch as bird life is closely connected with the life of cold blooded animals-insects, which the birds feed upon-and with the life of plants which not only furnish food but also form the environment, one can be sure that the climatic conditions on which the life and distribution of plants and of insects depend, must greatly influence the life of the birds and also determine their distribution. A correspondence can actually be seen between climatic zones and the distribution of a great number of bird species. Some birds are limited to the polar regions; such are the species belonging to the shores and islands of the Arctic Ocean and to the tundra (marshy Arctic lowlands). Others live only within the limits of the northern forest regions. Others again are limited to the southern steppes, and so forth. Nevertheless, investigation of the distribution of different bird species according to climatic zones, leads one to conclude that in the great majority of cases this distribution depends on the zonal distribution of the habitats to which this or that species is adapted. In other words, there exists a correlation between the distribution of birds and the zonal distribution of the stations or biotopes. I give the name ecotopycal factors to this category of ecological factors in order to distinguish them from climatic and biocenotic factors (Stantschinsky, '26). Birds possess a series of very perfect adaptations for the struggle against unfavorable climatic conditions which exist in moderate and in cold zones in winter. The majority of birds avoid this unfavorable season of the year either by regular flight to the south for the winter, or by migrating to a greater or lesser distance from their usual nesting places. Only a very few species of birds are completely sedentary, and, of course, it is upon them that the unfavorable climatic factors of winter existence act most strongly. However, the influence of climatic factors, owing to the birds' blood being warm, and their feather covering perfect, manifests itself directly on the bird distribution in a much lesser degree than it shows in the winter stations and biocenos. The water basins and marshes freeze up, a layer of snow covers the ground, plant vegetation ceases, animals with cold blood either perish or else hide or fall into hibernation. As yet we have scarcely any data for deciding the question of the degree