Changes in the composition of the urine of yellow-vented bulbuls (Pycnonotus xanthopygos): The effects of ambient temperature, nitrogen, and water intake

Uricotely (uric acid >50% of urinary nitrogen) in birds was once considered ubiquitous. However, Anna's hummingbirds (Calypte anna) have been shown to be an exception to this rule; under conditions of low ambient temperature (T_a) and on a nitrogen-free diet, they increased their water intake and often became ammonotelic (ammonia >50% of urinary nitrogen). Our aim was to identify the effects of nitrogen intake, water intake, and T_a on the ammonia excretion of yellow-vented bulbuls (Pycnonotus xanthopygos). We chose this predominantly frugivorous species because many of the characteristics of nectarivores that were used to explain increased ammonia excretion by C. anna are also characteristics of frugivorous birds. We assayed ureteral urine composition in eight yellow-vented bulbuls (P. xanthopygos), each randomly allocated a diet of 20% (0.6 M) sucrose solution supplemented by either 1.03 g/L or 7.23 g/L soy protein and held at a T_a of either 28°C or 10°C. Food, and therefore water, intake rates varied with nitrogen intake but not with T_a. Food intake increased significantly with decreased nitrogen intake, while concentrations of all the excretory compounds in the urine (P < 0.05) decreased; yet their proportions in the urine did not change significantly. The lower T_a had no significant effect on food intake or on the concentration of uric acid. However, at 10°C, the ammonia and urea concentrations increased (P< 0.05), and this led to a significant increase in the proportion of ammonia in the urine. Our results demonstrate that, when bulbuls are exposed to low T_a, they are able to save energy by increasing the proportion of the ammonia in their urine.