The Function of the Salt Gland in the Brown Pelican

It has long been a matter of speculation how oceanic birds cover their needs for water. Some marine birds remain at sea for weeks or months, hundreds of miles from land and any possible sources of fresh water. Sea water is known to be toxic to man and most other mammals, the reason being its high salt content. Some authors have stated that marine birds do drink sea water (Murphy, 1936: 337), while others have maintained that they can subsist wholly on water obtained from the food (Smith, 1953:163). In order to profit from the ingestion of sea water it is necessary for an animal to excrete salts in a concentration at least as high as that in the water ingested. The elimination of the salt would otherwise require an additional amount of water which would be taken from the body tissues. Therefore, if the organism cannot excrete a highly concentrated salt solution, the drinking of sea water only will lead to a progressive dehydration or a harmful accumulation of salt. The bird kidney is able to excrete salts in a concentration only about one-half of that found in sea water. Hence, if the bird kidney should excrete the salts from a given amount of sea water, it would be necessary to produce twice as much urine as the amount of water ingested. Thus, the kidney is not able to keep a marine bird in a favorable water balance if it drinks sea water.