THE HEAT INCREMENT OF FEEDING IN-HOUSE WREN CHICKS - MAGNITUDE, DURATION, AND SUBSTITUTION FOR THERMOSTATIC COSTS

The heat increment of feeding (HIF), a transient postprandial increase in metabolic rate, is the energy cost of processing a meal. We measur ed HIF in house wren chicks (Troglodytes aedon) ranging in mass from 1 .6 to 10.3 g. This mass range (age 2-10 days) spanned a transition fro m blind, naked, ectothermic chicks through alert, endothermic birds wi th nearly complete feathering. We fed chicks crickets (2.7-10% of chic k body mass) and determined HIF from continuous measurements of oxygen consumption rate (VO2) before and after meals. At warm ambient temper atures (T-a) of 33-36 degrees C, the magnitude of HIF (in mi O-2 or jo ules) was linearly related to meal mass and was not affected by chick mass. HIF accounted for 6.3% of ingested energy, which is within the r ange of results for other carnivorous vertebrates. The duration of HIF was inversely related to chick mass; 10-g chicks processed a standard meal approximately twice as fast as 2-g chicks. HIF duration increase d with increasing meal mass. The peak VO2 during HIF, expressed as the factorial increase above resting metabolism, was independent of body mass and meal mass. In large, endothermic chicks ( > 8 g), HIF substit uted for thermoregulatory heat production at low T-a.