Diel variations in plasma glucose concentration in some South African avian frugivores

We examined diel variations in the plasma glucose concentrations (P-Glu) of avian blood, by measuring P-Glu in four South African fruit-eating birds: the Cape White-eye (Zosterops virens), Knysna Turaco (Tauraco corythaix), Red-winged Starling (Onychognathus morio) and Speckled Mousebird (Colius striatus). Species were chosen as representatives of a range of body sizes and from the passerines and non-passerines. Birds were maintained in controlled environment rooms and P-Glu were measured at four times of the day, at sunrise (0600 hours), midday (1200 hours), sunset (1800 hours) and midnight (2400 hours). Interspecific variation in mass and both the interspecific and intraspecific variation in P-Glu were found to be significant for all species. P-Glu of Red-winged Starlings and Knysna Turacos were highest after overnight fasting, whereas peak P-Glu in Speckled Mousebirds and Cape White-eyes were recorded at midday. The lowest P-Glu was recorded after overnight fasting in Speckled Mousebirds whereas the lowest P-Glu were recorded at midnight in the three other species. Whereas the Red-winged Starlings, Knysna Turacos and Cape White-eyes clearly increase their P-Glu via the processes of gluconeogenesis, Speckled Mousebirds apparently do not. Previous studies involving the comparison of avian P-Glu have not specified sampling times and this study clearly demonstrates that P-Glu of birds vary over time, thereby highlighting the need for the specification of sampling times in future sampling of avian P-Glu.