Diurnal Variation in Mass, Metabolic Rate, and Respiratory Quotient in Anna's and Costa's Hummingbirds

Abstract

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 O2 consumption (V̇o2), CO2 production, respiratory quotient (RQ), and body mass in Anna's hummingbird, Calypte anna, and Costa's hummingbird, Calypte costae. During the day, V̇o2 was highly variable primarily because of differences in activity among individuals. At night V̇o2 varied little between individuals, but mean V̇o2 was more than two times that predicted from body mass for resting postabsorptive birds. For C. anna, mean 24-hr energy expenditure was similar to that of free-living birds. Diel mass fluctuations were large, up to 16% in both species. However, much of the observed change in mass was probably due to factors other than changes in body fat content. The RQ was well above 1.0 throughout the day, suggesting continuous deposition of fat, and RQs remained high at night (> 0.85), 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 for the observed nighttime RQs. This suggests that hummingbirds in this study were using their crop as a supplemental "energy storage depot" at night.