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Both technological and conceptual advances continue to enhance our ability to evaluate physiological mechanisms in free-living animals. Although complex and uncontrolled natural environments may challenge our ability to define causal mechanistic relationships, they provide opportunities not available in more conventional laboratory settings. Among these opportunities are the ability to observe the interplay between physiology and behavior, the potential inspiration to physiological studies from novel observations in the field, and the ability to evaluate the extent to which particular physiological systems are challenged under natural conditions. As we accumulate information about physiological function in the field, we are often forced to reconsider established paradigms: hibernating bears may contract their muscles to maintain strength and tone, testosterone levels in male stonechats maintaining territories in winter are exceptionally low, wintering emperor penguins may risk overheating, and large desert mammals may eschew brain-cooling mechanisms. Measuring and quantifying the organismal response to a changing environment provides a link between mechanistic physiology and behavior, ecology, and evolution and gives us new tools to understand population, community, and ecosystem-level processes.

Many birds switch seasonally or during ontogeny between diets of varying protein content. In mammals, high-protein diets induce hypertrophy of the kidney in general and of the thick ascending limbs (TAL) in particular, along with increases in glomerular filtration rate (GFR) and urine flow. A hypothesis to explain these phenomena is that the TAL become increasingly sensitive to peptide hormones (glucagon and antidiuretic hormone [ADH]) released in response to protein feeding; the consequent enhancement of ion reabsorption dilutes urine reaching the macula densa, thereby suppressing tubulo-glomerular feedback (TGF) and causing a rise in GFR. Avian kidneys possess most of the elements involved in this mechanism, including loops of Henle with TAL, sensitivity of TAL to ADH (arginine vasotocin [AVT] in birds), and the elements of TGF. We therefore hypothesized that switching from a low-protein to a high-protein diet would induce responses in birds similar to those found in mammals. We tested this hypothesis by feeding house sparrows, Passer domesticus, isocaloric diets containing either 8% or 30% protein. Birds on high-protein food had larger renal medullae, both in mass and in TAL diameter, but no increase in whole-kidney mass. Urine flow was approximately doubled on high-protein food, but there was no change in GFR. We were not able to detect an increased sensitivity of AVT-induced adenylyl cyclase activity in TAL from high-protein animals, and responsiveness to glucagon was higher in TAL from birds eating low-protein food. We are unable to conclude that a suppression of TGF is responsible for the rise in urine flow in birds eating high-protein foods, and the mechanisms behind the medullary hypertrophy and the diuresis remain to be fully explored.