Recent climate change has caused comparatively rapid shifts in the phenology and geographic distributions of many plants and animals. However, there is debate over the degree to which populations can meet the challenges of climate change with evolutionary or phenotypic responses in life history and morphology. We report that migrating birds captured at a banding station in western Pennsylvania, USA, have exhibited steadily decreasing fat-free mass and wing chord since 1961, consistent with a response to a warmer climate. This confirms that phenotypic responses to climate change are currently underway in entire avian assemblages. Declines in body size were not explained by an index of habitat condition within the breeding or wintering distributions. Instead, size was negatively correlated with temperature in the previous year, and long-term trends were associated with the direction of natural selection acting on size over the winter: species undergoing the strongest selection favoring small wing chord showed the most rapid long-term declines in wing. Phenotypic changes are therefore in line with the prevailing selection regime.