Climate change is forcing species to migrate to cooler temperatures at higher elevations, yet many taxa are dispersing slower than necessary. One yet-to-be-tested explanation for inadequate migration rates is that high-elevation environments pose physiological barriers to dispersal, particularly in species with high metabolic demands. Our global synthesis of >800 species supports this “physiological constraints” hypothesis: upslope migration is slower in insects that depend on nature’s most expensive locomotor strategy—flight.