Thermoregulatory use of heat increment of feeding in the tawny owl (Strix aluco)

The heat increment of feeding (HIF) was investigated in the tawny owl (Strix aluco) in central Norway (63 degreesN, 10 degreesE), close to the northern limit of its distribution. HIF was measured as the increase in heat production (measured as oxygen consumption) after force-feeding the owls with laboratory mice at thermoneutral conditions (20 degreesC) and during cold-exposure (5 degreesC and -5 degreesC). The basal metabolic rate of the owls (mean mass 419 g) was 4.39 kJ h(-1) and the lower critical temperature was approximately 16 degreesC. During cold conditions, HIF substituted for thermogenesis, and at an ambient temperature of -5 degreesC the substitution was complete. Calculations indicate that the substitution by HIF may save the owls as much as 60% of their daily thermoregulatory costs. This corresponds to about 10% of their total daily energy budget. (C) 2004 Elsevier Ltd. All rights reserved.