Nitrogen requirements of an old world nectarivore, the orange-tufted sunbird Nectarinia osea.

Nectarivorous birds are represented by three major radiations: honeyeaters and sunbirds in the Old World and hummingbirds in the New World. Costa's hummingbirds and New Holland honeyeaters have unusually low nitrogen requirements, which have been related to the species' low-protein, high-sugar diets. Therefore, we hypothesised that orange-tufted sunbirds (Nectarinia osea) would likewise have low-maintenance nitrogen requirements and low rates of endogenous nitrogen loss. To test this hypothesis, we measured nitrogen balance, total endogenous nitrogen loss, and body mass changes in captive birds, using insects as a nitrogen source. Nitrogen balance, estimated by regression analysis to be 3.9 mg d(-1), was less than one-half of that allometrically predicted, while total endogenous nitrogen loss (1.9+/-0.6 mg d(-1)) was less than one-third of the allometrically predicted value. Thus, orange-tufted sunbirds follow the same pattern of low nitrogen requirements found in hummingbirds and honeyeaters. Total endogenous losses of nitrogen in nectarivores are low because a fibreless, easily digestible liquid diet reduces nitrogen losses in the feces, while the protein-sparing effect of a diet containing largely sugar leads to low endogenous urinary nitrogen losses.