THE EVOLUTION OF HIGH SUMMIT METABOLISM AND COLD TOLERANCE IN BIRDS AND ITS IMPACT ON PRESENT-DAY DISTRIBUTIONS

Summit metabolic rate (M-sum, maximum cold-induced metabolic rate) is positively correlated with cold tolerance in birds, suggesting that high M-sum is important for residency in cold climates. However, the phylogenetic distribution of high M-sum among birds and the impact of its evolution on current distributions are not well understood. Two potential adaptive hypotheses might explain the phylogenetic distribution of high M-sum among birds. The cold adaptation hypothesis contends that species wintering in cold climates should have higher M-sum than species wintering in warmer climates. The flight adaptation hypothesis suggests that volant birds might be capable of generating high M-sum as a byproduct of their muscular capacity for flight; thus, variation in M-sum should be associated with capacity for sustained flight, one indicator of which is migration. We collected M-sum data from the literature for 44 bird species and conducted both conventional and phylogenetically informed statistical analyses to examine the predictors of M-sum variation. Significant phylogenetic signal was present for log body mass, log mass-adjusted M-sum, and average temperature in the winter range. In multiple regression models, log body mass, winter temperature, and clade were significant predictors of log M-sum. These results are consistent with a role for climate in determining M-sum in birds, but also indicate that phylogenetic signal remains even after accounting for associations indicative of adaptation to winter temperature. Migratory strategy was never a significant predictor of log M-sum in multiple regressions, a result that is not consistent with the flight adaptation hypothesis.