Summit metabolic rate (Msum, maximum cold-induced metabolic rate) is positively correlated with cold tolerance in birds, suggesting that high Msum is important for residency in cold climates. However, the phylogenetic distribution of high Msum 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 Msum among birds. The cold adaptation hypothesis contends that species wintering in cold climates should have higher Msum than species wintering in warmer climates. The flight adaptation hypothesis suggests that volant birds might be capable of generating high Msum as a byproduct of their muscular capacity for flight; thus, variation in Msum should be associated with capacity for sustained flight, one indicator of which is migration. We collected Msum data from the literature for 44 bird species and conducted both conventional and phylogenetically informed statistical analyses to examine the predictors of Msum variation. Significant phylogenetic signal was present for log body mass, log mass-adjusted Msum, and average temperature in the winter range. In multiple regression models, log body mass, winter temperature, and clade were significant predictors of log Msum. These results are consistent with a role for climate in determining Msum 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 Msum in multiple regressions, a result that is not consistent with the flight adaptation hypothesis.
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