O’CONNOR, T.P.; GIBBS, C.; KUROIWA, K.: Do New York house finches utilize metabolic seasonal acclimatization?

Winter in temperate climates represents a significant energetic challenge for small birds. Most species show a form of metabolic acclimatization that results in improved cold tolerance, although the specific mechanisms for this response remain unknown. Previous studies of house finches (Carpodacus mexicanus) in Michigan, Colorado, and California have found that the magnitude of metabolic acclimatization varies with the severity of the winter climate. We examined seasonal acclimatization in house finches in New York City and addressed the following questions: 1) Is there evidence of metabolic acclimatization in the form of seasonal changes in basal or maximal metabolic rate (BMR and MMR, respectively) in this population? 2) Does the mass of vital, and energetically costly, organs vary between winter and summer? 3) Are seasonal changes in MMR found during exercise (MMR-ex), or only in cold-stress (MMR-cs)? Winter (WN) house finches were captured in January, summer (SM) birds were captured in June, and all measurements were completed within 30 h of capture. MMR-ex was measured in a running/flying wheel and MMR-cs was measured using cold temperature in a helium-oxygen (helox) system. There was no seasonal change in body mass or BMR. Surprisingly, there was also no seasonal change found in MMR-cs or MMR-ex. Exercise tests yielded MMR values that were 25% greater than helox values. The mass of the heart, lungs, liver, and gizzard were significantly greater in WN than SM. Such changes are typically associated with increases in energetic demands, but all of our measurements of metabolism showed seasonal stability. In combination with previous studies, our results suggest that the magnitude of metabolic seasonal acclimatization in New York City house finches is modest, if it is present at all.