Reduction of pore area of the avian eggshell as an adaptation to altitude.

Standard measurements of water vapor conductance in units of mg of H(2)O . day(-1) . torr(-1) (SI equivalent is mg of H(2)O . day(-1) . pascal(-1)) of fresh eggs of the red-winged blackbird (Agelaius phoeniceus) and the native chicken of India (Gallus gallus) collected at altitude are significantly less than those of eggs of the same species collected near sea level. This decrease is caused by a reduction of the total effective pore area of the eggshell at altitude. It appears to be proportional to the reduction in barometric pressure and the simultaneous increase in the diffusion coefficient of water vapor. Thus, reduction in pore area offsets increased diffusivity at altitude, and water vapor loss through the eggshell at any altitude remains the same as at sea level. The data suggest a structural adaptation of the shell to altered diffusivity of gases at altitude in order to prevent excessive water loss of eggs during natural incubation.