Seasonal Variation of Vascular Oxygen Transport in the Dark-Eyed Junco

Small birds wintering in temperate climates must increase rates of aerobic metabolism to meet thermoregulatory demands. Increased oxygen delivery to the tissues and/or increased oxygen unloading at the tissues may be involved in meeting this need. This study examines seasonal variation in vascular oxygen transport of Dark-eyed Juncos (Junco hyemalis) in western Oregon. Oxygen dissociation curves, hematocrits, and oxygen carrying capacities were determined in summer- and winter-acclimatized juncos. Oxygen affinity did not vary seasonally, P50 values for summer and winter were 54.1 torr and 54.0 torr, respectively, at 41°C and pH 7.5 Hematocrit values averaged 47.2 ± 3.3% in summer and 52.6 ± 2.0% in winter and were significantly different. Mean values for oxygen carrying capacity, 13.33 ± 1.21 vol% in summer and 14.48 ± 1.20 vol% in winter, also differed significantly. Apparently, increased oxygen demands in winter-acclimatized Dark-eyed Juncos are met, in part, by increased oxygen carrying capacity, but not by decreased oxygen affinity.