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Ornithological Science
Abstract
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.
We are grateful to the directorate and researchers of The Joint Russian–Vietnamese Tropical Research and Test Centre (A. N. Kuznetsov, V. L. Trunov, I. V. Palko, Nguyễn Văn Khuê, Nguyễn Văn Thịnh and Vũ Mạnh) for providing rooms and facilities for work. We are grateful to V. Avilov, who provided us with long-term ambient temperatures at the study site. We thank E. A. Ershova and M. Brazil for assistance with English. We are grateful to A. Nord for advice on measuring body temperature in birds. We thank Reviewing Editor Y. Niizuma and two anonymous reviewers for their comments, which helped us to improve the manuscript. Financial support of the long-term field study was provided by the Tropical Centre and an RFBR grant (#18-04-00791a). The final data analysis was completed with the support of the Russian Science Foundation (RSF-FWO grant #20-44-01005).
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