Seasonal and Geographic Variation of Cold Resistance in House Finches Carpodacus mexicanus

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 Ta < −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 Ta < −60 C for a period that was significantly longer and shorter than the ones for winter house finches from southern California and Colorado, respectively. The labile character of cold resistance in house finches was further established by study of birds captured in southern California and maintained in an outdoor aviary in Ann Arbor, Michigan. After 6 mo in captivity these birds showed a level of cold resistance in January-February similar to that observed in free-living Colorado birds in winter. Evidence is assembled suggesting that winter acclimatization is primarily metabolic in captive as well as free-living house finches.