Wiktionary
northern birch mouse
n. A rodent in the Dipodidae family, (taxlink Sicista betulina species noshow=1).
Wikipedia
Northern birch mouse
The northern birch mouse (Sicista betulina) is a small rodent about 5 to 8 cm long (without the tail), weighing 5 to 13 g. It lives in northern Europe and Asia in forest and marsh zones.
It hibernates in underground burrows. It eats shoots, grains, berries, and sometimes insects.