Energy metabolism and body temperature in 13 sunbird species (Nectariniidae)

Abstract

1. 1. Diurnal cycles of body temperature, T b, and energy metabolism, M, at different ambient temperatures ( T a: +5 −+ 32°C) were tested in 13 sunbird species from various habitats and of different body masses (5.2–14.2 g) including one of the smallest passerines, Aethopyga christinae. 2. 2. Resting M-level (night) reaches T a- dependent mean values of 54% (+5°C) and 49% (+25°C) of activity M-levels (day). Expected level is ca 75%. 3. 3. Resting metabolic rate of sunbirds lies within the range of theoretically expected values for birds. 4. 4. Mean linear metabolism-weight regression of the night values follows: M = 0.102 × W 0.712 ( M = energy metabolism in kJ/hr and W = body mass in g). 5. 5. Thermal conductances, T c, are lower (−24%) than the predicted values. This is caused by a decrease of T b at low T a. Mean nocturnal T c is 3.2 J/g × hr × °C, mean day-time value is 4.3 J/g × hr × °C. 6. 6. The zone of thermoneutrality is, in most species, within a T a- range of 24–28°C. 7. 7. Normal day and night levels of T b are in the same range as reported for other birds of the same weight class. T b decreases slightly with falling T a (partial heterothermia). Lowest recorded T b was 34.2°C. 8. 8. No species tested showed any sign of torpor at night, independent of T a, body mass or habitat origin.