There has been rapid intraspecific trait change in many species over the past few centuries. While the impacts of such long-term changes on community functional structure remain unclear, it is vital to evaluate and predict responses of biodiversity and ecosystem function to global change. We examined intraspecific trait changes of 548 bird species from 86,539 specimens collected over the past 140 years and species composition data for 840 bird assemblages surveyed between 1969 and 2021 in North America. We found that rapid intraspecific trait changes have substantially impacted community-level trait composition and functional diversity of communities. Specifically, intraspecific downsizing has reversed the direction of change in community-weighted mean body size that would be expected from species composition change alone. Additionally, intraspecific changes of <3% across four traits have expanded community trait space and accelerated the rate of increase of functional richness across studied assemblages. Our findings highlight that intraspecific trait change is a key driver of community functional reorganization and should be considered alongside species composition change when assessing biodiversity responses to global change.