This study determined the interaction between energy assimilation and digestive constraints on the foraging behaviour of nectarivorous Gurney's sugarbirds (Promerops gurneyi), black sunbirds (Nectarinia amethystina), and malachite sunbirds (Nectarinia famosa) in the laboratory. Rates of sugar intake and consumption, transit time, and the concentration of sugar in the excreta were measured when birds were fed 0.25 mol L-1 sucrose, 0.73 mol L-1 sucrose, and 0.73 mol L-1 glucose. For each species, intake rates by volume were greater at low sugar concentrations, such that energy intakes per 90 min were similar, irrespective of diet, which supports the idea of regulated energy intake for these nectarivorous birds. All species were efficient at energy extraction, excreting 1% or less sucrose equivalent irrespective of the initial sugar concentration of each diet. Transit times of solutions ingested increased with an increase in sugar concentration. Birds maximized energy assimilation on high- and low-energy nectar diets by having high extraction efficiencies and short transit times. Sugarbird and sunbird foraging behaviours may be limited by digestive processes and the cost of carrying high-energy reserves as nectar sugar concentration increases.