Experimental Evidence for Temperature-Dependent Winter Lipid Storage in the Dark-Eyed Junco (Junco hyemalis oreganus) and Song Sparrow (Melospiza melodia morphna)

Abstract

Stored lipid is the main source of metabolic fuel during winter fasts of small birds, yet relatively little is known about the environmental cues that stimulate lipid storage. The ultimate-temperature hypothesis, well supported for a number of species, suggests that birds fatten in anticipation of average or record low winter conditions on a given day. In contrast, the proximate-temperature hypothesis suggests that small birds respond directly to temporal and spatial variation in winter temperature and deposit an amount of body lipid appropriate to current metabolic demands. Dark-eyed juncos (Junco hyemalis oreganus) and song sparrows (Melospiza melodia morphna) in southwest British Columbia were subjected to warm (20° C) and cold (20° C, gradually reduced to - 12° C over a period of days) temperatures for 10 d in enizironmental chambers allowing free flight and seminatural feeding and roosting conditions. Total lipid content was significantly greater (P < 0.025) in the cold group than in the warm group i...