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To examine how hummingbirds that do not enter torpor at night store and utilize energy, open-circuit respirometry and a strain gauge were used to measure daily variation in 0 2 consumption (Vo2 ), C02 production, respiratory quotient (RQ), and body mass in Anna's hummingbird, Calypte anna, and Costa's hummingbird, Calypte costae. During the day, Vo2 was highly variable primarily because of dif ferences in activity among individuals. At night Vo2 varied little between individuals, but mean Vo2 was more than two times that predicted from body mass for resting, postabsorptive birds. F0.85), indicating the use of carbohydrate as a metabolic substrate. Predicted crop volumes of the hummingbirds are sufficient to store the amount of feeder solution (0.25 g sucrose per ml) required to account/or the observed nighttime RQs. This suggests that hummingbirds in this study were using their crop as a supplemental "energy storage depot" at night.


Originally published in Physiological Zoology, 64:860-880, 1991.

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