Fig. 3. Metabolic rate of 2–3-day-old ducklings vs water temperature, adjusted to the average mass of 48 g. The slope of the lines indicates the effect of water temperature on metabolic rate. The spacing of the lines indicates the additional effect of air temperatures. The lines are drawn using the heat transfer model in Equation 3. Even with 30°C water temperature and 45°C air temperature, there is no indication that the thermal neutral zone has been reached.