Characteristics of Andean gull (Larus serranus) and Puna ibis (Plegadis ridgwayi) eggs laid at 4,400 mm in the Peruvian Andes were studied to determine how the conflicting requirements of maximizing O2 availability to the embryo while minimizing excissive losses of water vapor and CO2 have been met by avian populations breeding at high altitudes. Egg masses, linear dimensions, and surface areas of these montane eggs were similar to those of Heermann's gull (Larus heermanni) and glossy ibises (Plegadis falcinellus), but conductance to water vapor (G(H2O), standardized to 760 torr) of Andean gull and Puna ibis eggs averaged 74.5% and 68.4%, respectively, of lowland values. The difference in G(H2O) of Andean gull eggs was caused primarily by a reduction in the number of pores per egg; both an increased shell thickness and a smaller number of pores reduced G(H2O) of Puna ibis eggs. Since the reduction of G(H2O) of montane eggs did not fully compensate for the change in barometric pressure (59% of sea level) and the increase in gaseous diffusion coefficients at 4,400 m, the 'effective' conductance of the eggs at that altitude was greater than at sea level. Therefore, the eggs lost substantially more water during incubation than did lowland eggs. The modifications in eggshell characteristics of montane eggs may have resulted from selection to increase O2 availability to the embryo.