Differences between temperate and tropical birds in seasonal acclimatization of thermoregulatory traits

Phenotypic flexibility can be an important determinant of fitness in variable environments. The climatic variability hypothesis (CVH) predicts that phenotypic flexibility in thermoregulatory traits will be greater in temperate species than tropical species as a means of coping with increased temperature seasonality at higher latitudes. However, support for the CVH has been mixed, and recent studies suggest that tropical birds are capable of substantial phenotypic flexibility. To test the generality of the CVH, we used flow-through respirometry to quantify seasonal acclimatization in thermoregulatory traits in suites of temperate (n = 6) and tropical (n = 41) birds. We used W/S ratios (winter/summer trait values) to quantify the direction and magnitude of seasonal change (W/S ratio of 1 means no seasonal change). Temperate species exhibited coordinated changes in thermoregulatory traits in winter, including large increases in thermoneutral zone (TNZ) breadth and reductions in heat loss below the lower limit of the TNZ. Conversely, tropical species exhibited idiosyncratic seasonal thermoregulatory responses, and mean W/S ratios were close to 1 for all traits, indicative of little seasonal change and consistent with predictions of the CVH. Nevertheless, mean W/S ratios did not differ significantly between temperate and tropical species for either M_b or BMR, demonstrating that tropical birds can also exhibit substantial thermoregulatory flexibility. Our results highlight the need for complementary acclimation experiments to determine if latitudinal differences in seasonal acclimatization are due to inherent differences in capacity for flexibility.