Facultative hypothermic responses in an Afrotropical arid-zone passerine, the red-headed finch (Amadina erythrocephala).

We investigated thermoregulation and facultative hypothermic responses to food deprivation in the red-headed finch (Amadina erythrocephala), a 22-g passerine endemic to the arid regions of southern Africa. We predicted that, like most other passerines investigated, A. erythrocephala exhibits shallow rest-phase hypothermia, but not torpor. We observed significant reductions in rest-phase energy expenditure and body temperature (Tb) in response to restricted feeding. The maximum extent of Tb reduction (ca. 5 degrees C) and energy savings (ca. 10%) were consistent with those reported for a number of other passerine species. The lowest Tb we observed in a bird able to arouse spontaneously was 34.8 degrees C. The parameters of facultative hypothermic responses in A. erythrocephala were indicative of shallow rest-phase hypothermia, but not torpor. The limited available data on hypothermic responses in passerines suggest that many species do not possess the capacity for torpor. In passerines, torpor appears to be restricted to a few nectarivores and aerial insectarivores, and may have evolved independently of the torpor observed in non-passerine taxa such as the Trochiliformes and Caprimulgidae. The basal metabolic rate (BMR) of A. erythrocephala was 30-46% lower than predicted by various allometric equations, but was similar to the predicted BMR for a 22-g desert bird.