Seasonal Differences in the Response of Rufous Hummingbirds to Food Restriction: Body Mass and the Use of Torpor

Abstract

Daily food consumption of captive Rufous Hummingbirds (Selasphorus rufus) was restricted during two different seasons to determine the effect of chronically reduced food intake on body mass and the role of nocturnal torpor in promoting changes in body mass. Daily food consumption was reduced to 70-90% of ad libitum levels for 15 days in spring and for 30 days in summer. In spring, all birds (n = 4) showed increases in both body mass and use of torpor during restriction, although energy saved by using torpor was sufficient to compensate for energy lost through food restriction in only two birds. As soon as ad libitum feeding was resumed, three of the four birds showed a decline in body mass and a reduction in the use of torpor. In summer, birds (n = 9) had higher body masses at the start of the experiment, reflecting normal seasonal fluctuations in body mass. During food restriction, birds spent less time torpid than during spring restriction, and in most birds body mass declined or showed no significant change. The initial response of all birds to the return of ad libitum feeding included decreased use of torpor and, in contrast to spring, a rapid increase in body mass. Seasonal differences in response to food restriction may reflect reduced stress response in these high-altitude, high-latitude breeders during the short breeding season, when the physiological and behavioral consequences of responding to environmental stress may interfere with breeding success.