Avian eggs: barriers to the exchange of heat and mass.

Measured boundary-layer conductance to heat exchange for bird eggs varies with egg mass to the 0.53 power. Calculations based on the Nusselt-Reynolds relationship for a sphere and the thermal properties of air indicate that the conductance of the boundary layer to heat and to mass at any wind speed other than still air should scale with mass to the 0.53 power. Although the boundary layer contributes little to the total barrier to mass flux between bird eggs and their environment, we show that it is the major barrier to the exchange of heat. From these observations we infer that birds incubating eggs in natural nests can alter only the gradient affecting mass flux between their eggs and the environment while having the capability to change both the gradient and conductance affecting heat flux.