Can Seagulls Drink Salt Water? How Salt Glands Work

How a gull’s salt glands work

Seawater contains about 3.5% salt, much more than a bird’s blood. If a gull drank seawater and relied only on its kidneys, it would lose water trying to get rid of the salt, because bird kidneys cannot produce very salty urine. Salt glands solve the problem.

The two glands lie in shallow hollows in the skull above each eye, so they are often called supraorbital glands, meaning “above the eye socket”. Each gland is packed with tiny tubules surrounded by blood vessels. When salt levels in the blood rise after a gull swallows seawater or salty prey, nerves switch the glands on. Cells in the tubules pump sodium and chloride out of the blood and into the tubules, together with only a little water.

The result is a fluid that can be clearly saltier than seawater itself. It flows through ducts into the nasal cavity, out through the nostrils and along grooves in the bill to the tip, where it drips off or is flicked away with a shake of the head. In effect, the bird drinks seawater and gets rid of the salt in a smaller volume of water, keeping the rest. The glands do not turn seawater into fresh water; they remove the extra salt after it has been absorbed.

The physiologist Knut Schmidt-Nielsen and colleagues worked out how bird salt glands function in experiments in the late 1950s, which showed that the glands, not the kidneys, do most of the salt removal in seabirds.

Do seagulls prefer fresh water?

Yes. A herring gull can live on seawater, but it drinks and bathes in fresh water whenever possible, and gulls often fly inland to rain pools, rivers, lakes and reservoirs. Fresh water saves energy, because running the salt glands takes work.

Salt glands are also flexible. Experiments with ducks and other water birds have shown that the glands grow larger in birds living in salty conditions and stay smaller in birds living mainly on fresh water. Gulls that feed at freshwater lakes or in cities may rely on their glands much less than gulls living on offshore islands.

Which birds have salt glands?

All birds have nasal glands, but they are large and active mainly in birds that live at sea or eat salty food. These include:

• Gulls and terns, which feed on marine fish and invertebrates.
• Albatrosses, petrels and shearwaters, the tube-nosed seabirds. The salty fluid runs out through the tubes on top of the bill. The wandering albatross can spend years at sea and drinks seawater regularly.
• Penguins, which take in seawater with every meal.
• Pelicans and cormorants, including the brown pelican, which scoops up seawater along with fish in its pouch.
• Auks, such as puffins and guillemots, and sea ducks such as eiders.
• Flamingos, which feed in very salty lagoons and lakes.

In general, the more time a bird spends at sea and the saltier its food, the larger its salt glands. Ocean wanderers such as petrels and albatrosses have some of the most powerful glands of all.

Salt glands in other animals

Salt glands evolved several times in animals that live in or near the sea:

• Sea turtles release salt through glands near the eyes, which is why they seem to “cry” on nesting beaches.
• Marine iguanas of the Galapagos sneeze out salt from glands in the nose. See Why do marine iguanas sneeze?
• Saltwater crocodiles have salt-secreting glands on the tongue.
• Sharks and rays have a rectal gland near the end of the gut that removes extra salt.

Mammals have no salt glands. Whales, dolphins and seals get most of their water from their food and from fat breakdown, and they rely on very efficient kidneys.

Why people cannot drink seawater

Humans have no salt glands, and our kidneys are the only organs that can remove large amounts of salt. Even working at full capacity, human kidneys cannot make urine as salty as seawater. To get rid of the salt in a glass of seawater, the body has to spend more water than the glass contained, so drinking seawater makes dehydration worse, not better.

Seabirds avoid this trap because their salt glands release salt in a fluid that is saltier than seawater. Every sip therefore leaves a little fresh water behind for the body to use. Their kidneys then deal with other wastes and fine-tune the balance.

Myths and facts

Myth: Gulls’ salt glands turn seawater into fresh water. Fact: the seawater is absorbed first, and the glands then remove the extra salt from the blood.

Myth: A gull with a dripping bill is sick. Fact: a clear drip at the bill tip is normal and usually shows the salt glands working.

Myth: Seabirds never drink. Fact: many seabirds drink seawater regularly, and they also take in a lot of salt with their food.