Hyper-dominant species drive unexpected functional shifts in North American birds

Global environmental changes have led to widespread intraspecific shifts in functional traits across large numbers of widely distributed taxa. However, how within-species functional shifts predict community-level changes is unknown. We summarized seven key traits that are hypothesized to shift adaptively under environmental changes using local community-weighted means across North American bird communities. We find that communities shifted toward larger body mass and smaller appendage sizes over time, the opposite of widely observed within-species temporal changes in North American birds. These unexpected patterns arise from changes in the relative abundance of a very small number of hyper-dominant species, with 23 out of 470 (5%) species explaining the community-level trajectories in functional traits. These species are known to be heavily impacted--both positively and negatively--by human activities, and the idiosyncratic drivers of population sizes result in weak associations between community-level functional shifts and major bioclimatic variables but strong associations with human-induced land use changes. Our results illustrate the complexity in scaling from within-species responses to global change to community-level responses and highlight the importance of species-specific responses to the environment beyond climate when predicting functional and taxonomic diversity under global change.