Most bird eggs have evolved a suite of remarkably consistent adaptations for appropriate exchanges of respiratory gases and water vapor during incubation in nests above ground. However underground incubation is associated with selective forces different from those operating at the surface. New information from mound-building birds living reptiles and extinct dinosaurs shows comergent adaptations to nest atmospheres that are high in CO2 low in O2 and nearly saturated with water vapor. High humidity eliminates the danger of excessive dehydration so gas conductance of the eggshell may be higher than normal, as a compensation for the unusual nest gases. In contrast with embryos of birds that next above ground and initiate breathing inside the shell, those of megapode birds lack an aircell and can breathe only after the shell is broken. Extreme precocity of megapode chicks is related to long incubation time and large energy stores in the egg. Because the material around a buried nest restricts diffusion the size of the nest must be limited to prevent intolerable gas tensions adjacent to the eggs. This effect may have forced certain large reptiles to separate their layings into several clutches and some megapodes to actively ventilate their mounds.