The climatic variability hypothesis (CVH) posits that more flexible phenotypes should provide a fitness advantage for organisms experiencing more variable climates. While typically applied across geographically separated populations, whether this principle applies across seasons or other conditions (e.g., open vs. sheltered habitats) which differ in climatic variability remains essentially unstudied. In north-temperate climates, climatic variability in winter usually exceeds that in summer, so extending the CVH to within-population seasonal variation predicts that winter phenotypes should be more flexible than summer phenotypes. We tested this prediction of the within-season extension of the CVH by acclimating summer and winter-collected house sparrows (Passer domesticus) to 24, 5 and -10°C and measuring basal (BMR) and summit (M_sum = maximum cold-induced) metabolic rates before and after acclimation. To examine mechanistic bases for metabolic variation, we measured flight muscle and heart masses and citrate synthase and β-hydroxyacyl coA-dehydrogenase activities. BMR and M_sum were higher for cold-acclimated than for warm-acclimated birds and BMR was higher in winter than in summer birds. Contrary to our hypothesis of greater responses to cold acclimation in winter birds, metabolic rates generally decreased over the acclimation period for winter birds at all temperatures but increased at cold temperatures for summer birds. Flight muscle and heart masses were not significantly correlated with season or acclimation treatment, except for supracoracoideus mass, which was lower at -10°C in winter, but flight muscle and heart masses were positively correlated with BMR and flight muscle mass was positively correlated with M_sum. Catabolic enzyme activities were not clearly related to metabolic variation. Thus, our data suggest that predictions of the CVH may not be relevant when extended to seasonal temperature variability at the within-population scale. Indeed, these data suggest that metabolic rates are more prominently upregulated in summer than in winter in response to cold. Metabolic rates tended to decrease during acclimation at all temperatures in winter, suggesting that initial metabolic rates at capture (higher in winter) influence metabolic acclimation for captive birds.

House Sparrows acclimated to different temperature conditions (24, 5, and -10 C) for six weeks in both summer and winter.

Data collected included basal and summit metabolic rates (pre, mid and post-acclimation), ultrasound flight muscle width (pre and post-acclimation), and tissue masses and catabolic enzyme activities (post-acclimation).

There are scattered missing values for some of these variables. They appear as blank cells in the Excel file.