9307024 Beuchat Hummingbirds subsist almost entirely on a liquid diet composed of floral nectar, and when energy demands are high they can consume more than three times their own body mass in fluid per day. At the same time, however, the hummingbird's high metabolism requires efficient extraction of nutrients and electrolytes from a dilute food source that is passing rapidly through the gastrointestinal tract for efficient nutrient extraction, electrolyte conservation, and rapid elimination of water. For a hummingbird to simultaneously balance energy, water, and ion fluxes is a major challenge that raises fundamental questions about hummingbird morphology and physiology. This study proposes to address a central problem: In what ways and by what mechanisms do the kidney and gastrointestinal tract of hummingbird contribute to water and ion homeostasis while at the same time allowing maximum efficiency of energy acquisition? Using isotopically-labeled markers in artificial nectar, I can determine the fraction of ingested fluid that is absorbed by the intestine and the capacity of the kidney to eliminate excess water without depleting the body of electrolytes. The efficiency of energy and nutrient extraction are determined under conditions that influence the hummingbird's energy requirements, its electrolyte balance, and the quality of its diet. Together, these studies will provide information about the unique adaptations that allow hummingbirds to specialize on a liquid diet. ***
