Seasonal Adjustments in Metabolism and Insulation in the Dark-Eyed Junco

Trends during summer and winter in body mass and composition, insulation, body temperature, and metabolism were investigated in seasonally acclimatized Dark-eyed Juncos (Junco hyemalis) from western Oregon to determine if patterns of acclimatization were similar to those of passerines inhabiting harsher winter habitats. Body mass was significantly increased in winter (19.4 ± 0.2 g) compared to summer (17.8 ± 0.1 g), primarily because of elevated stored fat. Lean dry mass was seasonally constant. Dry mass of contour plumage was measured as an index of insulation and increased by 31.7% in winter birds. Little seasonal variation in body temperature was apparent and there was no evidence of torpor and only a very mild regulated nocturnal hypothermia at either season. Standard metabolic rate was significantly elevated in winter-acclimatized juncos (1.02 ml O2/min, 3.45 ml O2· g-1· hr-1) relative to summer-acclimatized birds (0.87 ml O2/min, 3.16 ml O2· g-1· hr-1). In addition, winter juncos demonstrated an ability to decrease thermal conductance below thermoneutrality. Insofar as winter increments in insulation do not offset increased thermogenic requirements at temperatures characteristic of this season, winter-acclimatization in Dark-eyed Juncos from western Oregon is similar to that for passerines from more severe winter climates in that it is primarily a metabolic process.