Added salt helps sunbirds and honeyeaters maintain energy balance on extremely dilute nectar diets.

Nectar-feeding birds ingest excess water and risk loss of solutes when they excrete it. Previous work has shown that white-bellied sunbirds (Cinnyris talatala) are unable to maintain energy balance on extremely dilute sucrose diets without salts (e.g. <0.25 mol l(-1)), and that they lose more electrolytes (i.e. Na(+) and K(+)) via cloacal fluid on these diets than on more concentrated diets. Using white-bellied sunbirds and New Holland honeyeaters (Phylidonyris novaehollandiae) we tested the effect of adding electrolytes to a 0.1 mol l(-1) sucrose diet, by including equimolar NaCl and KCl at concentrations from 5 to 40 mmol l(-1) and the individual salts at 20 mmol l(-1). Addition of salts enabled both species to drink significantly more of the 0.1 mol l(-1) sucrose diet than in the absence of salts, and mass loss during the experiment was reduced when salt was included. The larger honeyeaters may be more susceptible to electrolyte depletion than the smaller sunbirds. On 20 mmol l(-1) combined salts, both sunbirds and honeyeaters consumed eight times their body mass in fluid daily. KCl alone had no effect. Birds are thus limited in their consumption of extremely dilute diets by increasing losses of Na(+). This was confirmed by measuring plasma Na(+) levels, which decreased in both species in the absence of dietary Na(+). In addition, sucrose assimilation efficiencies were slightly, but significantly lower when sunbirds were fed salt-free diet, while glucose levels in ureteral urine remained extremely low. It is concluded that Na(+) depletion on very dilute salt-free diets does not affect Na(+)-glucose transport activity in the kidney, but interferes with sugar digestion and/or assimilation in the intestine.