Energetics of Incubation in Free-Living Orange-Breasted Sunbirds in South Africa

Abstract

Using the single-sample variant of the doubly labeled water method (DLW), I quantified the field metabolic rate (FMR) and water flux of female Orange-breasted Sunbirds (Nectarinia violacea) during the incubation period. Concurrent to DLW measurements, I examined their incubation behavior and microclimate. These are the first measurements of FMR and water flux for a nectarivorous bird during incubation and the first for an Old World sunbird species. For 10 incubating female Orange-breasted Sunbirds (mean mass = 9.5 g), CO2 production averaged 13.3 ml/hr, which corresponds to a FMR of 66.2 kJ/day, one of the highest values reported for a bird of its size. Among female passerines, the ratio of FMR to basal metabolic rate (BMR) is typically near 3.0, but for female Orange-breasted Sunbirds, the ratio of FMR/BMR is near 6.5. This suggests that during incubation females are working near their maximum capacity. Females generally lost mass during experimental periods; birds with high mass loss have elevated FMRs. Females consumed 14.7 ml H2O/day, slightly more than twice that expected based on body size. Attentiveness of females averaged 39.4 min/hr, while incubation bouts and inattentive periods averaged 11.9 min and 6.7 min, respectively. Females stayed away from their nests for long periods in the late afternoon, especially during inclement weather. Egg temperature averaged 34.7°C for all experimental periods combined. When females were absent for extended periods, eggs cooled to ambient temperatures, often near 10°C. This is a temperature well below the physiological zero temperature. The FMR of females increased significantly with decreasing operative temperature. One observation suggested that when in negative energy balance, females lower their body temperature while on the nest at night. These are the first data suggesting that sunbirds use hypothermia as an energy conservation mechanism in the field.