ENERGY-METABOLISM, RESPIRATORY QUOTIENT AND BREATHING PARAMETERS IN 2 CONVERGENT SMALL BIRD SPECIES - THE FORK-TAILED SUNBIRD AETHOPYGA-CHRISTINAE (NECTARINIIDAE) AND THE CHILEAN HUMMINGBIRD SEPHANOIDES-SEPHANOIDES (TROCHILIDAE)

1. The sunbird Aethopyga christinae and the hummingbird Sephanoides sephanoides have very similar body masses (5.2 and 5.7 g) and extensive morphological convergences. Experiments have been performed to determine whether both bird taxa also exhibit similar patterns of body temperature-regulation and energy metabolism. 2. The metabolic rate for the non-passerine hummingbird is higher than predicted but within the expected range for the passerine sunbird. The hummingbird undergoes torpor, the duration and level of which are clearly correlated with ambient temperatures. The sunbird does not show any lethargy even when forced to fast. 3. Under euthermic conditions body temperature (T(b)) in both species lies within the normal range. "Hypothermia" in the sunbird reduces T(b) to 32-33-degrees-C; T(b) of the hummingbird may fall slightly below 20-degrees-C during torpor. 4. The mean rate of heat loss below the thermoneutral zone is (day/night) 8.04/5.38 J(g h-degrees-C)-1 or 2.23/1.5 mW(g-degrees-C)-1 in the hummingbird and 8.61/4.21 J(g h-degrees-C)-1 or 2.39/1.17 mW(g-degrees-C)-1 in the sunbird. 5. The respiratory quotient at night varies between 0.75 and 0.80 in both species. Mean values during the day are 1.05 in the hummingbird and 1.2 in the sunbird. 6. The mean ventilation frequency (day/night) is 183/158 min-1 in the hummingbird and 193/117 min-1 in the sunbird, tidal volumes are 0.170/0.099 ml and 0.172/0.090 ml, respectively. Only minute volumes during night are significantly different between the species (higher in the hummingbird). 7. The hummingbird shows special physiological characteristics, whereas the data for the sunbird are almost in the theoretically expected range.