Concentration-dependent sugar preferences in nectar-feeding birds: mechanisms and consequences

1. Specialised nectar-feeding birds tend to prefer sucrose over glucose-fructose mixtures or to be indifferent when tested at concentrations close to 584 mmol L-1 sucrose equivalents. The role of the potential interaction between sugar composition and concentration on the sugar preferences of these birds, however, has not been explored. 2. We tested the hypothesis that sugar preferences in nectar-feeding birds are concentration-dependent. We predicted that at high concentrations they would prefer sucrose over hexoses, whereas at low sugar concentrations they would prefer hexoses. We expected birds to show differences in food intake that matched their sugar preferences when they fed on equicaloric solutions of sucrose and 1 : 1 mixture of glucose and fructose. Consequently, the curves describing the relationship between food intake and sugar concentration for these two sugar solutions should cross. We tested these hypotheses in two species of nectar-feeding birds: the Cinnamon-bellied Flowerpiercer, Diglossa baritula (Wagler) and the Magnificent Hummingbird, Eugenes fulgens (Swainson). 3. The sugar preferences of both species were concentration dependent. At lower concentrations they preferred hexoses, whereas at higher concentrations they shifted their preference to sucrose. However, these concentration-dependent preferences were not matched by parallel differences in intake. 4. Although nectar composition and concentration are often discussed as two different floral traits, our results show that they have a synergistic effect on the sugar preferences of nectar-feeding birds.