How Swans Stay White in Muddy Waters

They use oil from a special gland and their own saliva to keep dirt off their feathers

By Helen Czerski June 6, 2024 2:06 pm ET

Swans like this one in Hyde Park, London distribute dirt barriers from their beaks into their feathers. Photo: facundo arrizabalaga/Shutterstock

Swans are the gentle giants of my local waterways: floating paragons of snowy serenity that cruise peacefully through muddy brown water amid the rowdy confusion of their smaller brethren.

And they present a puzzle. Around them, mallards, Egyptian geese and Mandarin ducks have plumage so varied that it seems like any little speck of dirt or grime would disappear into the design. But the swans, paddling around in water so opaque that their feet can’t be seen, tip their tail feathers high in the air to forage underwater for the deepest plants—yet they re-emerge an unreasonably pure white. How do they stay so clean?

Part of the answer is that swans, like some other water birds, use ingenious physics and chemistry to prevent dirt from touching them in the first place. Each swan feather has a shaft with closely spaced barbs extending outward, and each barb has even smaller branches, or barbules, that interlock with each other to create a stiff, textured surface. Swans regularly comb their feathers with their beaks to keep these elements intact. But the feathers alone aren’t quite enough to keep out the marine elements. The real trick is in controlling a never-ending battle between oil and water.

Oil and water don’t mix; the two will always separate themselves out by repelling each other. We see that when we make a vinaigrette or when there’s a colorful sheen of oil on puddles in the street. The dirt in ponds and lakes is suspended in the water, so the swan’s solution is to coat itself in oil to keep the water away. This is called preening. Once the feathers are all tidily in place, swans use their long necks to reach around to a special gland just above the base of their tail—the preen gland. The gland releases an oily mixture that the swan distributes over its feathers, keeping them flexible and strong and making them very resistant to water. The technical term is “superhydrophobic,” and the dirt just slides off.

A pair of swans fly over a frozen pond in Bushy Park, London. Photo: John Walton/Associated Press

This all works beautifully, but what happens when bits of fatty or oily dirt come along? These tinier invaders aren’t repelled by the swan’s oils and find their way deep into the microstructure of the feathers. For that, micro-scale physics has a solution.

Just as a swan uses an oily coating to repel water, to get rid of oil it needs a watery coating. And the answer—discovered only three years ago in a paper published in the journal Advanced Functional Materials—is saliva. A swan’s spit is full of proteins that have a water-loving end and an oil-loving end. Once in a while, the bird distributes saliva on its feathers instead of preen oil, and the oil-loving ends stick to the feathers, leaving the water-loving ends exposed.

This makes the feathers attractive to water, and so allows the watery saliva to penetrate deep into the feathers. Once it’s there, it finds channels lined with tiny wedges. The wedge shapes help surface tension push the water from the center of the feather to the edges, sweeping along and clearing out any tiny droplets of oil or fat on the way.

The feathers get a watery deep cleaning as the tiniest oily contaminants are carried away. After a while, air dries the saliva out, and the surface returns to its normal water-hating state while the swan is restored to its pristine purity.

It’s a fascinating system, and scientists and engineers are now trying to replicate it to make self-cleaning fabrics that we could use. But kudos to the swan, for having evolved the perfect spit-and-sparkle system for keeping itself clean.

Appeared in the June 8, 2024, print edition as 'How a Swan Stays White in Muddy Waters'.