Meeting Abstract
Many avian species’ distributions are expected to shift with climate change, but detailed predictions are limited by our understanding of the physiological constraints that shape altitudinal range limits. As elevation increases, temperatures decline, requiring enhanced aerobic performance to sustain shivering thermogenesis, yet O2 availability declines in parallel. Birds adapted to life at high altitude harbor a number of unique physiological adaptations that enable superior O2 uptake, circulation, and utilization allowing them to maintain aerobic performance under hypoxic conditions. But to what degree are these traits flexible in lowland species, facilitating colonization of high environments? Dark-eyed Juncos (Junco hyemalis) breeding at high altitude winter at lower elevations, suggesting that they maintain physiological flexibility for these different environments. We measured juncos along a transect from 2070m to 3556m in elevation. The expected temperature reduction across this transect (10°C) should correspond to increases in both resting (RMR; 70%) and cold-induced summit (Msum; 19%) metabolic rates for summer-acclimatized juncos (per Swanson 1990, 1991). We documented little change in RMR (10% of the predicted rate; p = 0.08) while controlling for the acute effects of variation in PO2. Unexpectedly, Msum decreased with increasing elevation (β = 2.5 x 10-5 ml O2/g*min*m, p < 0.001). The reduced performance of highland birds suggests that acclimatization to hypobaric hypoxia may limit aerobic performance at moderate elevations in juncos. We explore the physiological modifications that might underlie this paradoxical pattern (e.g., blood parameters, cardiac and pulmonary physiology, body composition, and condition indices) and discuss the comparative selective forces of temperature and PO2 on avian physiological range limits.
