Ammonotely in a passerine nectarivore: The influence of renal and post-renal modification on nitrogenous waste product excretion

Most aquatic vertebrates are ammonotelic, whereas terrestrial vertebrates are typically uricotelic or ureotelic. However, the principal form of nitrogenous waste product in the urine of an animal may vary, depending on environmental conditions. Anna's hummingbird (Calypte anna) was found to switch from uricotely at high ambient temperature (T_a) to ammonotely at lower T_a, when energy demands and consequent nectar intake rates were high. In extension of this, we hypothesised that nectarivorous birds would switch from uricotely to ammonotely when water intake rates were high or when protein or salt intake rates were low. We examined the influence of water, electrolyte and protein intake and of T_a on the excretion of ammonia, urea and urate (uric acid and its salts) in nectarivorous Palestine sunbirds (Nectarinia osea). The proportion of ammonia in ureteral urine and excreted fluid was not influenced by total water or salt intake or by T_a. Protein intake did not influence nitrogenous waste product concentrations in ureteral urine. However, when protein intake was reduced, the proportion of ammonia in excreted fluid was higher because of the reduced urate concentration. This reduction in urate concentration leads to 'apparent' ammonotely. We suggest that ammonotely may not be a unique feature of nectarivorous birds. It could occur in any species in which breakdown of urate in the hindgut allows the uric acid-nitrogen concentration in the excreta to fall below that of the ammonia-nitrogen concentration.