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The house finch (Carpodacus mexicanus) is resident in tropical and subtropical regions as well as in localities having relatively severe winters. The extent of its winter acclimatization was assessed in freshly captured individuals of this species from southern California and Colorado. In severe cold stress tests involving exposure to T_a < −60 C, the former did not remain homeothermic any longer in winter than in late spring, whereas the Colorado birds did (8.8 vs. 97.5 min; P < .001). The capacity for winter acclimatization evident in these Colorado individuals was correlated with modest winter fattening, a response lacking in those from southern California. Freshly captured house finches from coastal Massachusetts were also studied in winter. These birds, whose ancestors were introduced to the Atlantic seaboard from California in 1940, remained homeothermic at T_a < −60 C for a period that was significantly longer …