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 measured 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 from blind, naked, ectothermic chicks through alert, endothermic birds with nearly complete feathering. We fed chicks crickets (2.7–10% of chick body mass) and determined HIF from continuous measurements of oxygen consumption rate (O₂) before and after meals. At warm ambient temperatures (T _a) of 33–36 °C, the magnitude of HIF (in ml O₂ or joules) 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 range 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 increased with increasing meal mass. The peak O₂ 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 substituted for thermoregulatory heat production at low T _a. Accepted: 11 December 1996