Fig. 1. Examples of changes in rates of oxygen consumption (_O2) associated with the voluntary ingestion of a single herring. (A) The bird was resting within its thermoneutral zone (TNZ) and fed a 100 g herring at time zero. (B,C) Birds were resting at sub-thermoneutral conditions (below 9°C) and fed a 100 g (B) or 60 g herring (C), respectively at time zero. The initial peak in _O2 is related to movements during the feeding event and was excluded (using linear interpolation, see broken line) when calculating HIF (indicated by the shaded area). The horizontal broken line denotes resting _O2, established from the stable period preceding feeding.