Energy and water balance in the lesser double-collared sunbird, Nectarinia chalybea

Abstract

Nectarivores feed on aqueous sugar solutions, and their water balance is tightly linked to their energy balance. When nectar is dilute and energy demands are high, consumption of a large excess of preformed water is inevitable. Physiological implications of nectarivory for the lesser double-collared sunbird, Nectarinia chalybea, have been investigated here. Sunbirds consumed 2.7 times their body mass (8 g) per day when feeding on 0.4 M sucrose at an ambient temperature of 10°C, and excreted 87 % of the water. When feeding on 1.2 M sucrose at 30°C, sunbirds drank only 0.5 times their body mass of water daily. In view of the sometimes high flux of water, combined with the low electrolyte concentrations of nectar, sunbirds must be efficient at conserving ions to maintainelectrolyte balance. When 15 mM each of KCl and NaCI were included in a diet of 0.4 M sucrose, sunbirds precisely maintained electrolyte balance by increasing cation excretion from 2 to 17 mM. Considering the high preformed water content of dilute nectar, sunbirds may expend large amounts of energy warming their food to body temperature, which was measured as 42°C irrespective of diet. Modelling revealed that sunbirds feeding on dilute (0.4 M) sucrose at 18°C would use the same proportion of their daily energy intake (4 %) to warm their food as high-latitude aquatic endotherms feeding on fish or invertebrates just above freezing point. The evaporative water loss (and therefore evaporative heat loss) of N. chalybea increased as dietary sucrose concentration decreased (more than two-fold between 1.2 and 0.2 M).