Fever is an energetically expensive component of the mammalian immune system's acute phase response. Like mammals, birds also develop fever when exposed to pathogens, but, as yet, the energy requirements of febrile mediation in birds are not known. We injected ducks (Anas platyrhynchos; n=8) with 100 mu kg(-1) LPS or sterile isotonic saline and recorded their core body temperatures while measuring their O-2 consumption and CO2 production in an open-flow respirometric circuit. Lipopolysaccharide elicited robust increases in the core body temperatures of our birds. The metabolic rate of the ducks increased about 80 min after treatment with LPS, relative to the metabolic rate of saline injected birds, and peaked 100 min later when the highest body temperatures were recorded. Our ducks increased their energy expenditure by 33.1% for about 3 h to mount a febrile response that, on average, increased their body temperature 1.4 degrees C. Studies with humans and rats, kept at thermoneutral temperatures, found a 10-15% increase in metabolic rate for every 1 degrees C increase in body temperature. The increase in metabolic rate, reported here (23%/degrees C), is noticeably higher and we conclude that febrile mediation is metabolically more expensive in Pekin ducks than in mammals. (C) 2011 Elsevier Ltd. All rights reserved.