Embryonic oxygen consumption, incubation period, and hatchling mass do not vary significantly among populations of Red-winged Blackbirds (Agelaius phoeniceus) breeding over a 2,900-m altitudinal gradient, despite a 28% decrease in oxygen tension from the lowest to the highest altitude. Calculations suggest that embryos develop normally at 2,900 m at air-cell 02 tensions of 7.1 kPa. The regulation of loss of water vapor or CO2 by a reduction of eggshell conductance appears to have been more important for populations breeding in montane environments than maximizing 02 availability to the embryo. The superior tolerance to hypoxia demonstrated by embryos of wild birds, compared to that of embryos of domestic chickens, may relate to the modification of the diffusive resistance of 02 inside the shell.