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Many birds reduce rest-phase energy demands through heterothermy, physiological responses involving facultative, reversible reductions in metabolic rate and body temperature (T_b). Here, we review the phylogenetic distribution and ecological contexts of avian heterothermy. Heterothermy has been reported in 140 species representing 15 orders and 39 families. Recent work supports the view that deep heterothermy is most pronounced in phylogenetically older taxa whereas heterothermy in passerines and other recently diverged taxa is shallower and confined to minimum T_b > 20°C. The reasons why deep heterothermy is absent in passerines remain unclear; we speculate an evolutionary trade-off may exist between the capacity to achieve low heterothermic T_b and the tolerance of hyperthermic T_b. Inter- and intraspecific variation in heterothermy is correlated with factors including foraging ecology (e.g …