By: Pat Yingst, Travis Audubon Master Birder
“Kidneys, Man!, Kidneys.” That is the punch line to a joke I heard in high school about some poor soul who confused his head with his kidneys. The joke popped into my mind when I learned that some birds have a gland above each eye which functions like an extra kidney.
All seabirds and many shorebirds have a pair of supraorbital (above the eye) glands which perform one of the kidney’s main functions: drawing salt ions out of the bloodstream. This salt gland allows these birds to drink the water they live in. Gulls, terns, petrels, albatrosses, grebes, knots, puffins, loons, penguins, pelicans, sea ducks and geese are some of the birds that possess salt glands.
The gland has a microstructure very similar to the kidney. It is made up of tiny capillaries which flow upwards separated from a vertical canal where salty secretions flow downward. The capillaries and the canal are separated by a wall that is only one cell thick. The cells in this wall are cells that specialize in absorption and secretion. At the base of the salt gland the vertical canal meets a horizontal canal which carries highly concentrated salt water secretions into the bird’s nasal passages where they drip out or are sneezed out of the body.
Birds, as well as other vertebrates like turtles or sharks or humans, cannot abide untreated salt water any more than people can. Drinking a quart of salt water will create a thirst which can only be slaked by drinking a quart and a half of fresh water. Without their salt glands, seabirds would die of thirst in their watery habitat.
The size of salt glands varies in birds and is related to species and to the salinity of the species’ habitat. There is some evidence that salt gland size can vary by individual and even mutate within one individual in accordance with the seasons and with migratory behavior.
Gulls and other shorebirds have grooves in their bills which allow the concentrated salty liquid to drip down the beak. You may see a gull with a liquid drop at the end of its beak. When the drop gets large enough the gull will shake its head to send the salty drop flying.
Birds in the order Procellariformes, which includes albatrosses, petrels, and shearwaters, have nostrils enclosed in double tubes which sit atop or along the sides of the upper mandible giving these birds the nickname “tubenoses.” These nostrils have multiple functions, one of which may be to allow the salty water to exit their bodies without falling back into their eyes as they fly.
One place you will not usually see salty secretions on birds’ beaks or nostrils is in bird photos. One of our Master Birder professors, Dr. Peter English, joked that bird photographers are notorious for photoshopping out such messy realities from their shots.
Coleridge in The Ancient Mariner writes, “Water, water, everywhere nor any drop to drink.”
Ah, it could be different if only we had extra kidneys in our heads!
Photo: Herring Gull, taken by Mike Mercer
