At high temperature (>40 ºC) endotherms experience reduced passive heat dissipation (radiation, conduction, and convection) and increased reliance on evaporative heat loss. High temperatures challenge flying birds due to heat produced by wing muscles. Hummingbirds depend on flight for foraging, yet inhabit hot regions. We used infrared thermography to explore how lower passive heat dissipation during flight impacts body-heat management in broad-billed (Cynanthus latirostris, 3.0g), black-chinned (Archilochus alexandri, 3.0g), Rivoli’s (Eugenes fulgens, 7.5g), and blue-throated (Lampornis clemenciae, 8.0g) hummingbirds in southeastern Arizona and calliope hummingbirds (Selasphorus calliope, 2.6 g) in Montana. Thermal gradients driving passive heat dissipation through eye, shoulder and feet dissipation areas are eliminated between 36-40 ºC. Thermal gradients persisted at higher temperatures in smaller species, possibly allowing them to inhabit warmer sites. All species experienced extended daytime periods lacking thermal gradients. Broad-billed hummingbirds lacking thermal gradients regulated mean total-body surface temperature at ~38 ºC suggesting behavioral thermoregulation. Blue-throated hummingbirds were inactive when lacking passive heat dissipation, so might have the lowest temperature tolerance of the four species. Use of thermal refugia permitted hummingbirds to tolerate higher temperatures, but climate change could eliminate refugia, forcing distributional shifts in hummingbird populations.