Evidence of Torpor in a Tropical Passerine, the Scarlet-Backed Flowerpecker Dicaeum cruentatum

Abstract The Scarlet-backed Flowerpecker Dicaeum cruentatum (Dicaeidae, Passeriformes), a sedentary species of Southeast Asia, is among the smallest passerine birds (5–6 g). Despite its very small size, it feeds mainly on plant foods, such as berries, nectar, and green seeds. We found that in conditions of likely food shortage Scarlet-backed Flowerpeckers were able to greatly reduce their metabolic rate at a relatively high ambient temperature (26–28°C). This is within the thermoneutral zone of most tropical birds, although slightly cooler than the normal lower critical temperature of flowerpeckers. In this state, the resting metabolic rate (RMR) of three individual, free-living Scarlet-backed Flowerpeckers from Vietnam averaged 1.57 mL O2/g*h, which was 3.4 times lower than their non-torpid RMR, measured at the same ambient temperature (Ta=∼27°C) and 2.5 time lower than their basal metabolic rate (BMR) measured at Ta=∼31°C. We did not measure the body temperature (Tb) of these individuals, but the dramatically low oxygen consumption provides evidence of torpor, an energy-saving physiological state, which is very rare among passerine birds. The skin Tb of active flowerpeckers, just before nocturnal RMR measurements, averaged 41.1°C (their cloacal Tb was 41.2°C), while in resting non-torpid birds at night the skin Tb averaged 36.3°C. Our report is the first quantitative evidence of torpor in the family Dicaeidae.