Sugar preferences in nectar- and fruit-eating birds: Behavioral patterns and physiological causes

Sucrose, glucose, and fructose are the three sugars that commonly occur ill floral nectar and fruit pulp. The relative proportions of these three Sugars in nectar and fruit in relation to the sugar preferences of pollinators and seed dispersers have received considerable attention. Based oil the research of Herbert and Irene Baker and their collaborators, a dichromy between sucrose-dominant humming-bird-pollinated flowers and hexose-dominant passerine flowers and fruits was proposed. Data oil sugar preferences of several hummingbird species (which prefer Sucrose) vs. a smaller sample of passerines (which prefer hexoses) nearly fitted this apparent dichotomy. This hummingbird-passerine dichoromy was strongly emphasized until the discovery Of Smith African plants with sucrose-dominant nectars, which arc pollinated by passerines that are able to digest, and prefer Sucrose. Now we know that, with the exception of two clades, most passerines are able to assimilate Sucrose. Most sugar preference studies have been conducted using a single, relatively high, sugar concentration in the nectar (ca 20%). Thus, we lack information about the role that sugar-concentration might play in Sugar Selection. Because many digestive traits are strongly affected nor only by sugar composition, but also by Sugar concentration, we suggest that preferences or different sugar compositions are concentration-dependent. Indeed, recent studies on Several unrelated nectar-feeding birds have found a distinct switch from hexose preference at low concentrations to sucrose preference at higher concentrations. Finally. we present some hypotheses about the role that birds could have played in molding the sugar composition of plant rewards.