Climatic adaptation in Svian standard metabolic rate.

The standard metabolic rate (SMR) of birds correlates broadly with climate of origin. SMR tends to be higher in birds from cold climates and lower in tropical forms than would be expected from the bird's mass. SMR changes, on the average, 1% per degree change in latitude. The influence of climate on SMR is, however, subject to modification by other aspects of the bird's life history. For example, in tropical species adaptive modifications in SMR correlate with thermal microhabitat. Tropical birds which forage in the sun have SMR's averaging 25% lower than expected, while SMR of species which forage in the shade is normal. Species of penguins which undergo prolonged fasts during the breeding season do not show elevated SMR's typical of high latitude birds.