Based on long-term studies (1968-2022), materials are presented on the change in the areas of birds in Eastern Siberia, caused by pronounced climate warming in the second half of the 20th - early 21st centuries. During the period of the last strong Pleistocene cooling, this territory was characterized by mountain-valley icing. As a result, the common areas of many bird species were torn apart. Subsequently, in isolated groups, evolution followed different paths, which led to the emergence of new closely related species. At present, due to the strong warming of the climate in Eastern Siberia (1.9°/100 years on average) and the change in the area boundaries of many bird species, the former gaps in their areas are being filled. For many bird species, secondary contact has been recorded with the formation of vast transitional zones in which their areas overlap to some extent. So far, there are relatively few of them, but the general trend towards overlapping areas of a number of bird species can be seen quite clearly. Among the species with area gaps without further speciation, shorebirds sharply predominate. However, among the species whose area gap ended in speciation, their number is significantly reduced. The current period of sharp climate warming and a significant change in the areas of birds in Eastern Siberia is characterized by a sharp increase in their biodiversity. However, it is combined with the low population density of many species, especially those new to the ecosystems of the region. Such a situation requires increased attention to the existing structure of the bird population and its dynamics, since it is observed extremely rarely - the shortest duration of the known centuries-old climate cycle is about 2000 years.

Based on many years of work (1968-2022) and the analysis of special literature, the main features of the modern avifauna of Eastern Siberia and its relationship with Pleistocene glaciations and the climate of the previous epoch are shown. The eastern regions of Russia and the adjacent more southern large mountain systems were characterized by mountain-valley glaciations in the Pleistocene. As a result, up to the Himalayas, the continuous ranges of birds of this period were torn into two parts, and, taking into account the very complex terrain of the territory, into a larger number of isolates. Gaps in bird ranges caused intensive speciation and, in comparison with the previous period, to the formation of increased diversity. Modern climate warming, especially pronounced in Eastern Siberia, has led to the intensive movement of birds to the north, as well as the formation of western and eastern bird settlement flows. As a result, the total diversity of the species composition of birds in Eastern Siberia increased by 116 species (23.6% of the original diversity). The basis of the new species are birds whose habitats were torn apart during the last Pleistocene glaciation. After the discontinuities of the ranges, the tendency to the formation of new species is clearly visible in forest and mountain bird species. In waterfowl and waterfowl, they often did not lead to the formation of new species. This is due to the fact that coastal birds often had a common range far in the south of Asia, where the overall climate cooling was the least. Currently, some species have already formed extensive zones of intergradation of the habitats of closely related species, and many have a clear general trend towards the development of this process. Consequently, mountain-valley glaciers dramatically increased the diversity and isolation of habitats and increased the intensity of speciation of birds based on new geographical isolates. Undoubtedly, prolonged cooling and warming of the climate in a complex and diverse geographical environment are environmental factors of the evolutionary process.