Fig. 6. Graphical representation of the effect of cold acclimation on the sustainable ambient temperatures. (A) Assuming that a values of 5x BMR represents an acceptable metabolic ceiling to heat production, and that the totality of the energy is spent in thermoregulation, then birds acclimated to our cold condition would have to face a temperature of -50.9°C to reach their ceilings. Warm-acclimated birds would attain this limit at -31.8°C. These values correspond to 72.2% and 64.8% of the maximal thermogenic capacity for cold- and warm-acclimated birds, respectively. Also shown is the equivalent heat production necessary to face the lowest average ambient temperature, 3°C, in the south Wadden Sea. (B) The energy expenditure needed to maintain a normothermic state under various ambient temperature faced by wintering islandica knots in the Wadden Sea. At 3°C, thermoregulatory costs accounts for 53.2% and 42.3% of the metabolic ceiling for warm and cold acclimated birds respectively. At -5°C, birds from the cold treatment would spend 50.9% of their sustainable energy expenditure in thermoregulation whereas individuals from the warm treatment would use 63.9% of sustainable metabolic rate in thermoregulation. These values are based on conductance estimates for a wind of 1 m s^-1 measured by Wiersma and Piersma (Wiersma and Piersma, 1994). See text for more details.