We measured background 2H and 18O abundances and field metabolic rate (FMR) for White-tailed Ptarmigan (Lagopus leucurus) above 3600 m elevation in the Colorado Rocky Mountains between May and July. 18O abundances ranged from 1982.4 to 2018.6 ppm [Formula: see text], while 2H abundance ranged from 142.8 to 154.0 ppm [Formula: see text]. Mean 2H abundance followed closely (−0.3 ppm deviation) the level predicted by Tatner's empirical model relating 2H and ambient temperature. However, 18O was more enriched than predicted (+3.4 ppm), which may reflect 18O fractionation in the plant diet. FMR, measured by means of the doubly labelled water method, ranged from 206.4 to 442.7 kJ/d and was not related to body mass. However, for males, FMR was significantly and positively related to age. Because of high variation in background isotope levels, the use of mean 2H and 18O background abundances instead of individual backgrounds would introduce a mean error of 7.1% (range −8.9 to +11.4%) in calculations of CO2 production and FMR.
