Osmoregulation by nestling and adult American Kestrels (Falco sparverius)

ABSTRACT.-We explored aspects of renal function in American Kestrels (Falco sparverius), birds that habitually eat high-protein meals. In particular, we hypothesized that, like granivorous birds switched to high-protein food, kestrels would have high urine flows with high proportions of waste N as urate. In nestlings in the field, 80-93% of urinary N was in the form of urate, though concentrations of ammonia and urea were higher in older nestlings. Feeding resulted in reduced osmotic and ion concentrations in urine, but concentrations and proportions of nitrogenous wastes were unaffected. In adult kestrels in the laboratory, urine flow rate in fed animals, ~1 mL h^sup -1^, was similar to that previously measured in avian granivores. Urine flow declined with fasting, accomplished by both a reduction in glomerular filtration rate (significant after 24 h fast) and a rise in tubular water reabsorption (significant after 48 h). During the course of a 48 h fast, proportion of urinary N excreted as urate fell and that as ammonia rose. Both filtration and secretion of urate dropped during fasting, so that tubular secretion remained responsible for ~90% of urinary urate. Composition of fluid voided from the cloaca differed little from that of the ureteral urine. Received 10 August 2000, accepted 27 November 2001.

RESUMEN.-Exploramos aspectos relacionados con la funcion renal en el halcon Falco sparverius, un ave que usualmente consume alimentos con alto contenido proteico. Particularmente, hipotetizamos que tal como en aves granivoras que cambian a una dieta de alto contenido proteico, los halcones deberian tener altos flujos de orina con una alta proportion de desecho de N en forma de urato. En polluelos en el campo, el 80-93% del N urinario se encontro en forma de urato, aunque las concentraciones de amoniaco y urea fueron mas altas en polluelos de edad mAs avanzada. La alimentacion condujo a una reduccion de la concentracion ionica y osm6tica de la orina, pero las concentraciones y proporciones de desechos nitrogenados no fueron afectadas. En halcones adultos en el laboratorio, el flujo de orina en animales alimentados (~1 mL h^sup -1^) fue similar al previamente medido en aves granivoras. El flujo de orina disminuyo con el ayuno, lo cual se manifesto tanto en la reducion de la tasa de filtracion glomerular (significativa despues de 24 h de ayuno) como en el aumento de la reabsorcion tubular de agua (significativo despues de 48 h). Durante un periodo de ayuno de 48 h, la proporcion de N urinario excretada como urato disminuyo y la excretada como amoniaco aumento. Tanto la filtration como la secrecion de urato disminuyeron durante el ayuno de manera que la secrecion tubular permanecio dando cuenta de aproximadamente un 90% del urato urinario. La composicion del fluf do excretado por la cloaca difirio poco del excretado por la uretra.

IN GRANIVOROUS BIRDS, a switch from a lowprotein diet to a high-protein diet influences a number of aspects of kidney structure and function. Increased excretion of nitrogenous wastes, particularly urate (uric acid and its salts), is one obvious consequence of elevated dietary protein (e.g. Ward et al. 1975). In addition, urine flow is elevated (Wheeler and James 1950, McNabb et al. 1972, Ward et al. 1975) and the renal medulla may hypertrophy (Goldstein et al. 2001). Little is known, though, of renal function in birds that habitually consume diets high in protein content. Only a few studies of carnivorous species have examined aspects of urine composition (e.g. Calder and Bentley 1967, Skadhauge 1974, McNabb et al. 1980, Hughes 1984, Janes 1997). Measures of other aspects of renal function, such as rates of glomerular filtration or urine flow, have been accomplished only in marine carnivores (Oelofsen 1973, Hughes 1980, Goldstein 1993). In the present study, we examined aspects of renal function in a small falcon, the American Kestrel (Falco sparverius; m^sub b^ == 125 g). This species is not known to use a nasal salt gland for excreting dietary ions (Cade and Greenwald 1966), and so relies on its kidneys, and possibly cloaca, for osmoregulation. Kestrels consume a variety of prey in the wild, including insects and small vertebrates. They must curtail hunting during certain periods of inclement weather, and by reducing metabolic rate can probably tolerate at least five days of fasting (Shapiro and Weathers 1981). We predicted that the high protein diet of kestrels would entail high rates of urine flow and urate excretion. On the other hand, we also hypothesized that the combination of relatively small size and adaptation to periodic fasts would have led to the evolution of welldeveloped capacities for water and energy conservation. Thus, we predicted substantial change in renal function in the transition from eating to fasting. We tested these ideas by measuring kidney function in captive adult kestrels, and also by examining urine composition in free-living kestrel chicks.

MATERIALS AND METHODS