Salt glands and Sea Water: Duck Farming in 1960s Hong Kong and a Giant Petrel in Argentina

Salt-glands enable many birds to drink saline waters and eat salt-rich invertebrate foods. A question that arises is the extent to which the salt gland is used in birds that can fly or swim to a source of fresh water to drink. Do they just stay in salt water and get rid of the salt through the salt glands, or do they, if possible, go to fresh water to drink? Salt glands—just like man-made desalination plants—need a great deal of energy to run, so one might predict that birds with salt glands would, if they could, drink fresh water. There could be a trade-off, of course, between using energy to fly or swim to fresh water and using energy to stay put with the salt glands working.

Several weeks ago I wrote about an article by V.C. Wynne-Edwards in a magazine of the 1930s—before the discovery of salt glands in the 1950s. He had found that gulls do not, in general, stray too far from a source of fresh water and suggested that they only drink there.

The first actual observation I heard of was in Hong Kong in the 1960s. Domestic ducks and geese have salt glands and given salty water to drink can switch them on in a matter of minutes. They cannot, however, unlike truly marine birds, survive on full-strength sea water for any length of time.The late John Phillips (1933-87) was appointed Professor of Zoology at the University of Kong Kong in 1962 at the age of 29 . He had previously worked on salt glands for a while and soon after his arrival in Hong Kong had been driving through the New Territories when he saw a large flock of domestic ducks dabbling for food on salt flats near Yuen Long. He thought the salt glands must have been well and truly working in those birds. That may indeed have been the case, temporarily at least, since although mechanical arrangements in the beak can reduce the ingestion of water along with food, the ducks must have been taking in some water nearly as salty as sea water. However, as he continued watching the farmer appeared and herded the ducks back to his land. The ducks ran onto the farm and headed straight for a container of fresh water which they drank avidly.

I had been planning to write this article but lacked a photograph of a flock of domestic ducks in Hong Kong. Then Rob Taylor posted this photograph in a Facebook Group on Hong Kong in the 1960s; I show it here with his approval. The ducks appear to be sitting on the bank of a gei wai—an artificial salt water pond filled by the waters of the Pearl River estuary and used for rearing salt-water shrimps.

Domestic ducks- New Territories, Hong Kong.
Photograph from Rob Taylor's family collection

I saw something similar in in a very different setting with a very different species. New to the birds of the southern ocean, I was keeping my eye on a Southern Giant Petrel (Macronectes giganteus) as we walked along the shore in Ushuaia in southern Argentina. The wind was icy and as I pulled a scarf around my mouth—I had raging toothache—I noticed the petrel moving close inshore. It made for a drain from which fresh water after a heavy shower was flowing into the sea. The bird had a long drink from the drain and headed paddled off to sea. 

Thus in both the domestic ducks and the petrel, fresh water would have diluted any excess salt in the body and enabled excretion of dilute urine through the kidneys rather than concentrated salt from the salt glands.

These observations on birds drinking fresh water in preference to salt water accord with experiments that have been done. Most birds tested—even a number that have salt glands—prefer fresh water to salt water when offered a choice. A possible exception is an albatross that is truly oceanic.