Cold tolerance and thermogenic capacity in dark-eyed Juncos in winter: Geographic variation and comparison with American tree Sparrows

Abstract

1. 1.Cold tolerance and maximal thermogenic capacity ( V ̇ O 2 max ) under cold stress in helox gas mixtures (approx. 79% He, 21% O 2) were determined for Dark-eyed Juncos ( Junco hyemalis) and American Tree Sparrows ( Spizella arborea) wintering in SE South Dakota. Junco data were compared to previous data on juncos wintering in the milder climate of western Oregon. 2. 2.Dark-eyed Juncos from South Dakota (SDJU) tolerated colder helox temperatures than Oregon birds (ORJU). This improved cold resistance was not associated with increased thermogenic capacity or SMR in SDJU, as SMR and total V ̇ O 2 max did not differ significantly and mass-specific V ̇ O 2 max was greater in ORJU. 3. 3.SDJU were significantly larger and had significantly lower thermal conductance than ORJU and these may be important to improved cold tolerance capabilities in the former. 4. 4.SDJU showed significantly higher total V ̇ O 2 max than sympatric American Tree Sparrows (ATSP), but mass-specific V ̇ O 2 max did not vary as juncos were significantly heavier than sparrows. 5. 5.SDJU tolerated slightly colder temperatures than did ATSP. Thermal conductance and SMR did not differ between the two species. Sympatric wintering Dark-eyed Juncos and American Tree Sparrows, which show similar ranges and habits, also demostrate similar metabolic performance in the cold.