Since the middle of the last century, there has been an increase in numbers and an intensive range expansion of the mute swan, Cygnus olor, to the north and northeast. The species is dispersing into different types of landscape: on shallow eutrophic lakes, as well as on sea moraine and selga islands. To date, the range of the species in northwestern Russia covers the eastern part of the Gulf of Finland, the western part of Leningrad oblast, all of Pskov oblast, and the western parts of Novgorod and Tver oblasts. However, in the past 30 years, the rate of expansion has significantly decreased, this suggesting the presence of climatic barriers to its further dispersal. We analyzed the relationship between the current distribution of nesting mute swans on inland water bodies in northwestern Russia and the main temperature and ice factors (duration of the ice-free period, timing of ice melt on inland lakes, mean January and annual temperatures). The analysis was carried out using binomial generalized linear models (GLMs). Isolines of climatic factors were constructed using kriging in the program Surfer environment from Golden Software. Despite the statistical significance of all the variables considered, ice variables (duration of the ice-free period and timing of ice melt on lakes) were shown likely to render a direct influence on the distribution of the species in northwestern Russia, because in other parts of its range the mute swan can breed at much lower temperatures than in the study area. It seems that it is the duration of the ice-free period that severely limits the reproduction success of the mute swan, as its reproductive cycle takes about 7–7.5 months. However, in choosing a breeding site in spring, birds are likely to be guided by the timing of ice melt, which should be not later than the time of the decline in their mating activity.
Kouzov et al. (Mon,) studied this question.