Fig. 1. Experiments consisted of three phases. In phase 1, broad-tailed hummingbirds (Selasphorus platycercus) were fed on a C₃ diet for roughly 3 months. Phase 2 began on day 0 when the isotopic composition of the diet was changed from C₃ to C₄. Phase 2 lasted 20 days. Phase 3 began on day 20, when the isotopic composition of the hummingbirds' diet was shifted back to the original C₃. Vertical lines denote the days we measured ¹³C of exhaled CO₂ in fasted and fed birds. We collected exhaled air from hummingbirds that were lightly restrained within a centrifuge tube that had been previously flushed with CO₂-free air.