Pollination by passerine birds: why are the nectars so dilute?

Bird-pollinated flowers are known to secrete relatively dilute nectars (with concentrations averaging 20-25% w/w). Many southern African plants that are pollinated by passerine birds produce nectars with little or no sucrose. Moreover, these hexose nectars are extremely dilute (10-15%). This suggests a link between sugar composition and nectar concentration. Nectar originates from sucrose-rich phloem sap, and the proportion of monosaccharides depends on the presence and activity of invertase in the nectary. Hydrolysis of sucrose increases nectar osmolality and the resulting water influx can potentially convert a 30% sucrose nectar into a 20% hexose nectar, with a 1.56 times increase in volume. Hydrolysis may also increase the gradient for sucrose transport and thus the rate of sugar secretion. When sucrose content and refractometer data were compared, some significant correlations were seen, but the occurrence of sucrose-rich or hexose-rich nectars can also be explained on phylogenetic grounds (e.g. Erythrina and Protea). Hexose nectars may be abundant enough to drip from open flowers, but evaporation leads to much variability in nectar concentration and increases the choices available to pollinators.