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Plasma glucose concentrations in birds are much higher than those in mammals of similar body mass, and they are thought to be kept more or less constant. We investigated plasma glucose concentrations of Malachite Sunbirds (Nectarinia famosa) to determine whether there was a circadian rhythm in plasma glucose and whether plasma glucose concentrations rose at lower temperatures. We explored the possibility that glycogenolysis and gluconeogenesis might be used by birds before daylight to generate body heat during arousal from torpor or from overnight adaptive heterothermy. Plasma glucose concentration of Malachite Sunbirds were relatively high, between 13.6 and 21.4 mmol/L. Plasma glucose concentrations were higher at 5°C than at 25°C, and generally lower during the scotophase, particularly in the early hours of the morning. Therefore, it appears that Malachite Sunbirds are not increasing plasma glucose concentrations during arousal from torpor or to increase scotophase body temperature levels to photophase levels. It is apparent that these nectarivorous birds have a circadian variation in plasma glucose concentrations that is affected by ambient temperature and feeding patterns.

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Sugar preferences and digestion of sugars in artificial fruit of different sugar types and concentrations were investigated in a South African facultative frugivore, the Cape white-eye (Zosterops virens). We studied sugar preferences, daily food and energy intake, and digestive efficiencies and transit times with birds that were fed artificial fruit diets comprising 6.6, 12.4 and 22% glucose and sucrose, respectively. Digestive transit rates of birds fed artificial fruit diets were faster for glucose compared with sucrose diets, irrespective of concentration. Birds showed increased food intake with decreasing glucose concentration, but showed no significant differences in these intake amounts with differing sucrose concentrations. Apparent assimilation efficiencies were lower than most nectar studies, and were generally higher for glucose fruit (79.3-85.6%) compared with sucrose fruit (69.0-78.4%). In choice tests, birds preferred sucrose over glucose of equivalent weight, irrespective of concentration. It appears likely that the Cape white-eye has high levels of sucrase activity, although further study is needed to verify this.

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