I tested the hypothesis that hypoxia in tree cavities causes the unusually short incubation periods characteristic of woodpeckers. I compared saturated O₂ levels in ambient air to O₂ levels in 13 unoccupied and 14 occupied nest cavities of the Northern Flicker (Colaptes auratus). Oxygen in unoccupied cavities did not differ from ambient levels, whereas O₂ in cavities with an incubating adult was 2.2% lower than in saturated air. However, the magnitude of the decrease (20.5% oxygen in occupied cavities compared to 21.0% O₂ in saturated air) is small compared to variation in the partial pressure of O₂ caused by altitude. This suggests that O₂ depletion is not a major selective force on embryonic development times for birds nesting in tree cavities. Alternative hypotheses for short incubation periods of woodpeckers include sexual selection on females and sibling competition.