Temporal and spatial environmental variation shapes organisms, populations and ecosystems. In birds, global warming and climate gradients across large spatial extents alter the properties of different annual-routine processes, including reproduction, molt and migration. Juveniles of most passerines species replace their nest-grown plumage partially during the first months of their life, a process that is called post-juvenile feather molt. This molt may largely determine the individual appearance, influencing, for example, bird attractiveness, social status and camouflage. Using field-data from several localities in the Palearctic regions, as well as museum data from 11 natural history collections, we show that the extent of the post-juvenile molt (number of feathers molted) has increased significantly over the last 200 years, a trend that is positively correlated with the temperature of the environment. In addition, we found that among passerines, post-juvenile molt extent differs between the Western and the Eastern Palearctic zones. This difference is most likely the result of a large-scale climatic gradient in cold season duration and consequently the time available for molting. Our results indicate that birds replaced more feathers under warmer conditions, causing juveniles to appear more similar to adult birds. These results highlight the analogy between temporal and spatial responses to climate differences, and as such may improve our understanding of various impacts of global climate change on avian life-history properties and the evolution of annual-routine scheduling of different processes such as breeding, molt and migration.