1. Global climate change affects many aspects of biology and has been shown to cause body size changes in animals. However, suitable datasets allowing the analysis of long-term relationships between body size and climate are rare. 2. The size of the skull, often used as a proxy for body size, does not change much in fully grown vertebrates, but some soricine shrews shrink their skull and brain in winter and regrow it in spring. This is thought to be a winter adaptation in these high-metabolic, nonhibernating animals, as a smaller brain size reduces energy requirements. 3. Climate could thus affect not only the overall size but also the pattern of the size change, i.e., Dehnel’s Phenomenon, in these shrews. 4. We assessed the impact of the changes in climate on the overall skull size and the different stages of Dehnel’s phenomenon in skulls of the common shrew, Sorex araneus, collected over 50 years in the Białowieża Forest, NE Poland. 5. Overall skull size decreased, along with increasingly mild winters and decreasing soil moisture, which determined the availability of the shrews’ main food source, earthworms. The magnitude of Dehnel’s phenomenon increased over time, indicating an increasing selection pressure on animals in winter. Overall, climate clearly affected the common shrew’s overall size as well as its seasonal size changes. With the current acceleration in climate change, the effects on the distribution range of this cold-adapted species may be quite severe.