Megapode

Abstract: Synopsis. 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 dif? ferent from those operating at the surface. New information from mound-building birds, living reptiles and extinct dinosaurs shows convergent adaptations to nest atmospheres that are high in C02, low in 02 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.

Abstract: The beginning and the end of the breeding season of the mallee fowl, Leipou ocellata Gould, are determined by the availability of natural heat for incubation. The breeding is confined to the summer because that is the only time when sufficient heat is available. The clutch size is a function of the length of the period suitable for egg-laying and the interval between successive eggs. The former is determined largely by the amount of winter rainfall and by the composition and location of the mound. The interval between eggs is affected by the amount of food available to the birds. Clutch size, then, is ultimately determined by the food supply of the female. Quantitative data on the size of mallee fowl eggs are presented and seasonal trends in egg size are discussed.

Abstract: 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 cm³ O₂/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 ($E_{i}$) based on O₂ consumption closely approximate estimates based on energy content of fresh eggs and hatchlings. The $E_{i}$ 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% (M...