Abstract

Body size of individuals is typically increasing towards colder and higher latitudes, as larger body size is known to be a thermoregulatory adaptation to colder circumstances. Here we explored the spatio--temporal changes in wing length and body weight of 24 wintering bird species in Northern Europe and how these relate to annual changes in winter temperature. We show that across all species, body size has decreased since the 1970s, with a negative relationship between wing length and temperature anomalies of previous winters, suggesting carry over effects likely linked with body size--related survival or dispersal. Body weight was negatively related with the temperature anomaly of the same winter, indicating more immediate effects related to reduced fat reserves during mild winters. Our results highlight climate--driven decrease in body size across several species and its' association with annual variation in winter temperature in the high latitudes.