CALORIMETRIC INVESTIGATIONS ON MOUND-BUILDING BIRDS

Megapodes, and mound-building birds of the Pacific, incubate their egg s underground where the heat is supplied by organic decomposition, sol ar radiation, or geothermal sources. The incubation biology of the Aus tralian mound builders has been examined from the standpoints of (1) t he energetic requirements of malleefowl (Leipoa ocellata) to construct and maintain incubation mounds and (2) the physics, physiology, and b ehavior concerning mound thermoregulation of the brush turkey (Alectur a lathami). The energetic cost of mound tending in malleefowl is estim ated by indirect calorimetry. The power required for digging into the mound to reach the eggs is approximately 20 W, or 3.8 times the basal metabolic rate of the bird. About 850 kg of sand has to be removed and replaced on the mound every time it is opened, a task requiring about 5 h. The overall energy investment for incubation for the entire 9 mo nth season is 2.5 times that expected in similarly sized birds. Brush turkey mounds become homeothermic because of (1) the high thermal iner tia of their 3-11 t mass and (2) a stable equilibrium that is reached between heat production and heat loss. The mound tends to seek the equ ilibrium, but the bird adjusts the temperature by subtle manipulation of the mound. At an equilibrium temperature of 33 degrees C, the mound produces about 110 W. Empirical data from natural mounds, set in the context of a numerical model of heat production and heat flux in the m ound, indicate that equilibrium temperature is extremely sensitive to ambient temperature and mound size, but not water content. Wet or dry mounds can maintain appropriate incubation temperature, but the dry on es have low thermal conductivity, retain the heat well, and minimize t he amount of forest litter collected during the incubation season.