Mallee fowl (MF) and brush turkey (BT) lay large, energy-rich eggs (173 g at 10.2 kJ/g contents for MF and 180 g at 9.8 kJ/g contents for BT) that are incubated by burial in mounds of warm earth or decaying vegetation. Their incubation periods of 62 days (MF) and 49 days (BT) are unusually long, and their hatchlings are among the most precocial of any birds. Metabolic rates of embryos of both species just prior to hatching are about 61 cm' 02/h, 98% (MF) and 64% (BT) higher than predicted. Metabolism is supported solely by chorioallantoic respiration until hatching, and pulmonary respiration begins suddenly when the shell membranes are torn. Estimates of total energy expenditure during incubation prior to hatching (Ei) based on 02 consumption closely approximate estimates based on energy content of fresh eggs and hatchlings. The E1 are high (-600 kJ for MF and ~-475 kJ for BT) because of the long incubation periods. Hatchlings must dig out of the incubation mounds, and costs of this may add 8% (MF) to 33% (BT) to the E, before hatchlings reach the surface. Lower emergence costs and larger initial energy stores of MF eggs partly compensate for higher developmental costs, so chicks of both species begin life on the surface with similar energy reserves. The large yolks (>50% of egg contents) and high energy content of megapode eggs were essential adaptations in the evolution of a reproductive system in which embryonic development and hatchling behavior are energetically expensive.