To the population ecology of the birch mouse (Sicista betulina Pall.) at the northern limit of the range. Communication II. Reproduction, ecological structure of the population, population dynamics

The results of a long-term (1959-2017) study of reproduction, age-sex structure and dynamics of the birch mouse population in Eastern Fennoscandia are summed up. It was established that under these conditions, birch mice mature and begin to reproduce at the age of about one year, after the first wintering. The study of the estrous cycle of females showed that on the northern periphery of the range, the birch mouse is a monocyclic species that only once per summer comes to the state of estrus, and the estrus of females coincides with the active spermatogenesis of males in time. Under the conditions of our region, adult mice breed from the beginning of June to August, bringing only one brood of 5.1 + 0.2 cubs, with fluctuations from 3 to 11, during this period, on average for 53 females. The age composition of the population varies from season to season. Until the end of July, almost exclusively adult overwintered individuals are caught in traps and grooves, and starting from the first ten days of August, an increasing share of the catches is occupied by arrived animals. Due to the determination of the age of mice from the layered structures of the lower jaw, it was possible to determine that the basis of the group of overwintered animals is formed by individuals born in the previous year (80 %), followed by mice that survived two winters (16 %), and individuals three years and older are very rare (4 %). Thus, the number of each next age group decreases in comparison with the previous one by 5-6 times, or on average by 80 %, which is the average annual mortality of this species.