Field Metabolic Rate and Food Consumption by Free-Living Anna's Hummingbirds (Calypte anna)

We measured CO2 production and water flux using doubly labeled water in wild Anna's hummingbirds living in the Santa Ana Mountains of Southern California during autumn (September) of 1981. The estimated field metabolic rate (FMR) of a hummingbird maintaining a constant body mass (mean 4.48 g) is about 32 kJ/day, which is 5.2 times basal metabolic rate (BMR). Metabolic rates during daylight hours were about 6.8 × BMR, less than one-half that expected for an Anna's hummingbird in continuous hovering flight. We estimated nighttime metabolism to be near 2.1 × BMR, which is about what would be expected for a normothermic, resting bird experiencing cool air temperatures (as low as 15 C) but much higher than expected if torpor were employed. Water influx was about 164% of body mass per day in birds maintaining a constant mass. Most of this water intake was in the form of sucrose solution from feeders in the area, but some probably came from insects eaten by the birds. Hummingbirds probably did not drink liquid water from streams or ponds during the measurement period.