Extrarenal Salt Excretion in Birds

Abstract

Investigations on cormorants ( Phalacrocorax auritus) gave no evidence to support the hypothesis that sea birds must drink sea water in order to cover their normal needs for water. When the birds were fed fresh fish the water content of the fish was more than adequate for the renal elimination of salts and nitrogen (uric acid). However, it was found that when given a salt load cormorants also employ an extrarenal mechanism for electrolyte elimination. Whether the load is imposed orally or by infusion of hypertonic NaCl solution, cormorants excrete a highly hypertonic liquid that drips out from the internal nares and collects at the tip of the beak, from which the birds shake the drops with a sudden jerk of the head. The concentration (500–600 mn NaCl) and the rate of secretion (up to 0.2 ml/min. in a 1.5-kg bird) are so high that with continuous secretion the entire NaCl content of the body could be eliminated in roughly 10 hours. The secretion contains practically only sodium and chloride in nearly equivalent amounts. The production of the nasal secretion is stimulated also by a nonelectrolytic osmotic load (sucrose), which indicates that the mechanism responds to general osmotic conditions rather than specifically to the sodium or chloride concentration of the plasma. The nasal secretion has been observed only after an osmotic load and never in fasting birds or after ingestion of fish or fresh water.