SUGAR DIGESTION EFFICIENCIES OF GURNEYS SUGARBIRDS, MALACHITE SUNBIRDS, AND BLACK SUNBIRDS

This study determined the interaction between energy assimilation and digestive constraints on the foraging behaviour of nectarivorous Gurne y's sugarbirds (Promerops gurneyi), black sunbirds (Nectarinia amethys tina), and malachite sunbirds (Nectarinia famosa) in the laboratory. R ates of sugar intake and consumption, transit time, and the concentrat ion 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. Fo r each species, intake rates by volume were greater at low sugar conce ntrations, such that energy intakes per 90 min were similar, irrespect ive of diet, which supports the idea of regulated energy intake for th ese nectarivorous birds. All species were efficient at energy extracti on, excreting 1% or less sucrose equivalent irrespective of the initia l sugar concentration of each diet. Transit times of solutions ingeste d increased with an increase in sugar concentration. Birds maximized e nergy assimilation on high- and low-energy nectar diets by having high extraction efficiencies and short transit times. Sugarbird and sunbir d foraging behaviours may be limited by digestive processes and the co st of carrying high-energy reserves as nectar sugar concentration incr eases.