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Climate change has led to shifts in phenology, food availability and increasing temperatures, affecting ecosystems and organisms. According to Bergmann’s rule, endotherms, such as birds, are expected to develop a smaller body size in a warmer climate. At the same time, insect populations seem to decline, causing food shortage that may primarily affect insectivorous birds and leading to a reduction in body size. Increasing temperatures could also prolong growing seasons leading to increased food availability later in the season and change the timing of migration. By using bird ringing data collected over 15 years (2010-2024) of six passerine species from the Ume River Delta Field Station the study examined if: (i) weight and wing length in fall migrating birds have decreased, (ii) changes in weight and wing length are general or if insectivores and seed eaters differ, and (iii) the timing of fall migration has changed over the past 15 years. The results showed a significant effect for diet on change in body size and migration over time. In general, the weight increased, wings became shorter, and the migration was delayed. The changes appear to be mainly driven by longer growing seasons and extended availability to food and are most likely an indirect consequence of a warmer climate. There was also a great variation in change within both diets and species, indicating that the causes are multiple and complex and have not yet been fully explained or understood.