LIKNES, E.T.*; SWANSON, D.L.: Seasonal Metabolic Acclimatization in the American Goldfinch Revisited: To What Extent Does Summit Metabolism Vary Seasonally?
Large seasonal changes in summit metabolic rate (Msum) have been considered only a minor component of winter acclimatization in small birds. This conclusion is based largely on data from Cardueline finches, including American Goldfinches. Recent evidence from a wider diversity of species suggest that relatively large winter increments of Msum (25-50%) are common among small passerines wintering in cold climates and are associated with improved winter cold tolerance. In this study, we evaluate seasonal changes in cold tolerance, BMR, and Msum for American Goldfinches from southeastern South Dakota. Goldfinches were captured in winter (Jan-Feb), spring (April), and summer (Jun-Aug) and tested on the day of capture. Cold exposure tests involved subjecting individual birds to a decreasing series of temperature in 79%He/21%O2 (helox) concurrent with open-circuit respirometry. The helox temperature eliciting hypothermia was designated the cold limit (Tcl). Winter goldfinches demonstrated significantly higher BMR (46%) and Msum (34%) and significantly lower Tcl (-9.5 vs. 1.3�C) than their summer counterparts. April goldfinches also showed significantly higher Msum (24%) and significantly lower Tcl (-5.3�C) than summer birds. Winter birds had higher BMR (23%, P<0.05) and Msum (9%, P=0.066) than April birds. Winter birds also had significantly lower Tcl than April birds. These data indicate that prominent winter increases in Msum are a component of winter acclimatization in goldfinches from South Dakota. Furthermore, seasonal changes in Msum accompany seasonal changes in cold tolerance, suggesting that Msum and cold tolerance are functionally linked.
