Water relations of buried eggs of mound building birds

Summary

Eggs of the brush turkey (BT) and mallee fowl (MF) are incubated in mounds of soil and plant litter. Humidity in BT mounds is always near saturation (>99% RH), but in MF mounds it drops to lower values in summer (x=77% RH). Despite these high humidities, the eggs lose an average of 9.5% (BT) and 12.0% (MF) of their initial mass by evaporation before hatching. The rate of evaporation$(\dot M_{{\text{H}}_{\text{2}} ^{\text{O}} } )$ increases during incubation several-fold due to large changes in water vapor conductance of the shell$(G_{{\text{H}}_{\text{2}} ^{\text{O}} } )$ and embryonic heat production. Values of$G_{{\text{H}}_{\text{2}} ^{\text{O}} }$ in fully incubated eggs in mound material are about 3–6 times higher than values obtained from unincubated eggs in desiccators. This effect depends on two factors: (1)$G_{{\text{H}}_{\text{2}} ^{\text{O}} }$ increases with ambient humidity, especially above 80% RH, possibly because the effective site of evaporation moves out along the walls of the pores in the eggshell. (2) Structural changes of the pores due to calcium absorption by the embryo directly increase$G_{{\text{H}}_{\text{2}} ^{\text{O}} }$. The first factor is most important in BT eggs and the second is dominant in MF eggs. Production of metabolic heat by the embryo increases the vapor pressure difference across the shell and further increases$\dot M_{{\text{H}}_{\text{2}} ^{\text{O}} }$, especially in mounds of high humidity. The changes in pore structure are adaptive because they produce high conductances to respiratory gases and cause normal gas tensions within the egg at the end of development, yet$G_{{\text{H}}_{\text{2}} ^{\text{O}} }$ is low enough in early development to prevent excessive water loss. Water not lost by evaporation or taken up by the embryo is stored and released during hatching. A small amount of mass is lost during incubation by respiratory gas exchange.