Lotz, C.N.: Effects of nectar concentration on water balance, osmoregulation, and thermoregulation in a nectar-feeding sunbird

The water balance of nectar-feeding animals is inextricably linked to their energy balance. Because nectar secreted by bird-pollinated flowers can be dilute, birds can be forced to consume high volumes of water to satisfy their energy demands. I examined the consequences of feeding on dilute nectar for the African sunbird Nectarinia chalybea (Nectariniidae). Sunbirds ingested the same amount of sugar over a wide range of sugar concentrations by modulating volumetric intake. When feeding on 0.2 M sucrose, birds consumed four times their body mass in water daily. Most of this water was excreted, with a remarkably small loss of electrolytes. Evaporative water loss was twice as high in sunbirds fed 0.2 M sucrose than in those fed 1.2 M sucrose. I estimated that birds used 10 % of their total daily energy intake to warm 0.2 M sucrose from 20 oC to body temperature, in contrast to the negligible amount of energy used to warm 1.2 M sucrose. To compensate for the increasing costs of evaporative heat loss and food warming with decreasing food concentration, sunbirds maintained higher metabolic rates while feeding on lower nectar concentrations. I hypothesize that the thermal insulation of sunbirds feeding on dilute nectar is higher than that in birds feeding on more concentrated nectar, because the increase in metabolic rate that I observed accounts only partially for the estimated costs of evaporative heat loss and food warming.