4.3. Measurements of metabolic rate

Heat production was quantified with indirect calorimetry by measuring oxygen consumption either with a S-3A oxygen analyser (Applied Electrochemistry Inc.) (I–IV) or with a Servomex 1440C oxygen analyser (V) in a metabolic chamber. Oxygen consumption (O₂) was calculated according to Hill (1972):

Equation 4.

where is flow rate through the chamber, m is body mass, F_in is fractional oxygen content (0.20953) of the dry CO₂-free inlet air, and F_out is oxygen content of the dry CO₂-free outlet air. For calculation of the respiratory exchange ratio (RER), carbon dioxide production was also measured with a Servomex 1440C carbon dioxide analyser (IV). Oxygen consumption or carbon dioxide production was recorded either with a pen recorder (I, II) or with a Hewlett-Packard 34970A Data Acquisition/Switch unit connected to a PC (III, IV, V).

The metabolic rate was measured in either one or several constant ambient temperatures (I, IV, V) or during a slowly decreasing ambient temperature (II, III, IV). For the measurements, birds were resting in a non-postabsorptive state (I–V) or else metabolic rate was manipulated in part of the birds with either cold-acclimation (III), or fasting (IV) or forced bipedal exercise on a treadmill (V).