Understanding avian diet preferences reveals a great deal about the birds' digestive physiology and relationships with food plants, and can make a valuable contribution towards directing physiological and ecological research. Importantly, diet preferences are likely to reflect physiological constraints and therefore mechanisms of digestion. We assessed the interaction between diet concentration and sugar-type preferences of three Australian nectarivorous bird species. Each individual bird was offered paired energetically-equivalent diets: a sucrose solution and hexose (1 : 1 mixture of glucose : fructose) solution over a range of diet concentrations from 0•075 to 2 mol L−1 Sucrose Equivalents (SE). Similar patterns were found for all three species. Intake on the most dilute diets was insufficient to maintain energy balance, suggesting that these birds faced physiological constraints on such diets.
All three species demonstrated a preference for hexose over sucrose when offered dilute diets, and sucrose (or none) preference on more concentrated diets. The three species differed in terms of when this switch from hexose to sucrose preference took place. Rainbow lorikeets (Psittacidae, c. 135 g body mass) demonstrated hexose preference for diets up to and including 0•75 mol L−1 SE; sucrose was preferred on 2 mol L−1 SE diets. Red wattlebirds (Meliphagidae, c. 105 g) showed hexose preference on only the most dilute (0•075 mol L−1 SE) diet, and sucrose preference on 1 and 2 mol L−1 SE diets. New Holland honeyeaters (Meliphagidae, c. 22 g) preferred hexose on 0•075 and 0•1 mol L−1 SE diets, and their selectivity for sucrose was not statistically significant. We suggest that the switch from hexose preference may be directly related to the digestive capacity of different taxa. Accumulating evidence suggests similar patterns of sugar preferences in various nectarivorous bird lineages. A switch from hexose preference on dilute diets to sucrose preference on concentrated diets has now been shown for hummingbirds, flowerpiercers, sunbirds, honeyeaters and lorikeets. Hexose preference on dilute diets suggests that reduced digesta retention time and low sugar concentration influences sucrose hydrolysis efficiency, whilst absorption rate of monosaccharides is less limiting. Sucrose preference on concentrated diets is more puzzling, but may reflect preference for diets with lower osmolality. Varying preferences suggest that the co-evolutionary relationships between birds and nectar sugar composition are likely to be similarly dynamic and situation dependent.