Seasonal Changes in Resting Metabolic Rate and Mass-Specific Conductance in Svalbard Ptarmigan, Norwegian Rock Ptarmigan and Norwegian Willow Ptarmigan

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Abstract
Resting metabolic rate was measured at different ambient temperatures in Svalbard Ptarmigan, Norwegian Rock Ptarmigan and Norwegian Willow Ptarmigan at different times of the year. All three species/subspecies exhibited a significant decrease in mass-specific resting metabolism from summer to winter. At both seasons resting metabolism was higher than predicted by Aschoff and Pohl (1970), but lower than the predictions of Weathers (1979) for all three species/subspecies. The mass-specific conductance (MSC) was significantly (P < 0.05) lower, i.e. insulation was better, in winter than during summer in Svalbard Ptarmigan and Willow Ptarmigan, while Rock Parmigan did not show seasonal changes in MSC. MSC of all three species/subspecies was lower than predicted both by Herreid and Kessel (1967) and by Aschoff (1981), both in summer and in winter. MSC of Svalbard Ptarmigan was significantly lower than MSC of Rock Ptarmigan and Willow Ptarmigan, both in summer and in winter. During spring (March-May) when the Svalbard Ptarmigan is still in winter plumage, but devoid of subcutaneous fat, the MSC was the same as during summer. This indicates that the subcutaneous deposits of fat contribute significantly to the thermal insulation of these high-arctic birds during winter.