Salt intake and regulation in two passerine nectar

Authors: Cromwell Purchase Susan W Nicolson Patricia A Fleming

Publish Date: 2012/12/12

Volume: 183, Issue: 4, Pages: 501-510

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Abstract

Avian nectarivores face the dilemma of having to conserve salts while consuming large volumes of a dilute electrolytedeficient diet This study evaluates the responses to salt solutions and the regulation of salt intake in whitebellied sunbirds Cinnyris talatala and New Holland honeyeaters Phylidonyris novaehollandiae Birds were first offered a choice of four sucrose diets containing no salt or 25 50 or 75 mM NaCl The experiment was repeated using five sucrose concentrations 0075–063 M as the base solution Both species ingested similar amounts of all diets when fed the concentrated base solutions However when birds had to increase their intake to obtain enough energy on the dilute sucrose diets there was a general avoidance of the higher salt concentrations Through this diet switching birds maintained constant intakes of both sucrose and sodium the latter may contribute to absorption of their sugar diets A second nochoice experiment was designed to elucidate the renal concentrating abilities of these two nectarivores by feeding them 063 M sucrose containing 5–200 mM NaCl over a 4h trial In both species cloacal fluid osmolalities increased with diet NaCl concentration but honeyeaters tended to retain ingested Na+ while sunbirds excreted it Comparison of Na+ and K+ concentrations in ureteral urine and cloacal fluid showed that K+ but not Na+ was reabsorbed in the lower intestine of both species The kidneys of sunbirds and honeyeaters like those of hummingbirds are well suited to diluting urine however they also appear to concentrate urine efficiently when necessaryThis project was funded by the National Research Foundation of South Africa the University of Pretoria and the Australian Research Council The Gauteng Directorate of Nature Conservation granted permits to capture and house the sunbirds and the Australian Department of Environment and Conservation approved our use of honeyeaters All animal care procedures and experimental protocols adhered to institutional regulations of Murdoch University R1137/05 and the University of Pretoria EC01307

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