How Do Ducks Stay Dry? Waterproof Feathers Explained

Feathers built like a zipper

The secret starts with the structure of a contour feather, the neat outer feathers that give a duck its shape. Each feather has a central shaft with rows of branches called barbs on each side. Each barb carries even smaller branches, the barbules, and the barbules on one side carry tiny hooks that latch onto the barbules of the next barb.

The result is a continuous, flexible vane, a bit like fabric held together by millions of microscopic zippers. On a duck, the vanes overlap like roof tiles across the body. Water landing on this surface meets a dense mesh of fine fibers with tiny air gaps between them. Surface tension stops the water from pushing through the gaps, so it sits on top as round beads and rolls away.

Waterbirds such as ducks, geese and swans have especially dense body plumage, with many feathers packed close together. That density is a big part of why a mallard can sit on a pond all day and still have dry skin.

The preen gland and preening

The second part of the system is the preen gland, also called the uropygial or oil gland. It sits on the duck’s back, just above the base of the tail, and produces a waxy oil.

A duck twists its head back, squeezes oil from the gland with its bill and then works through its plumage feather by feather. Preening does several jobs at once:

• re-zips the barbs that have come apart, closing gaps where water could get in
• spreads oil, which keeps feathers supple, stops them from becoming brittle and adds to their water-repellence
• removes dirt, old feather sheaths and parasites such as feather lice

Is it the oil or the structure?

Many people assume the oil alone makes ducks waterproof. Ornithologists generally find that the feather structure does most of the work: a clean, intact feather repels water well, while a damaged one leaks even when oiled. The oil is best thought of as maintenance, keeping the feather structure in good condition and adding some water-repellence on top. This is why ducks spend so much time preening; on many days a duck devotes hours to it.

Down: the warm, dry underlayer

Under the contour feathers lies a thick layer of down. Down feathers have no hooks and no stiff vane; instead their fluffy barbs form a soft, springy cluster that traps air. Still air is an excellent insulator, so as long as the outer coat keeps water out, the down stays dry and holds body heat in.

Ducks control this insulation actively. Fluffing the feathers increases the air layer on cold days, while pressing them flat helps a bird dive or lose heat. Mallard hens even pluck some of their breast down to line the nest, which keeps the eggs warm while she is away feeding.

Trapped air also adds buoyancy. That is why dabbling ducks such as mallards float high on the water, while diving ducks press air out of their plumage before going under.

How ducklings stay dry

A newly hatched mallard duckling is covered in dense down and can swim within a day. Its down sheds water reasonably well, and ducklings have their own preen gland from an early age, so the old idea that ducklings are not waterproof until they rub against their mother’s oily feathers is a myth.

Ducklings are still vulnerable, though. Their down is not as effective as adult feathers, and they lose heat quickly because they are small. In the wild, ducklings spend time out of the water and are regularly brooded under the hen’s body to warm up, especially in their first week or two. As the down is replaced by proper feathers over the following weeks, they become fully waterproof.

When the system fails

A duck’s dry coat depends on clean, intact feathers, so anything that disrupts the structure can let water in.

• Oil spills coat feathers in sticky oil that collapses the fine structure. Water then soaks through to the skin, and oiled birds can quickly become dangerously cold.
• Detergents and soaps lower the surface tension of water, letting it slip through gaps it would normally bridge. Scientists have shown that even small amounts of detergent can make feathers wettable.
• Damage and molt open gaps in the plumage. During the summer molt, mallards are flightless for a few weeks and stay close to cover while new feathers grow.

Not all waterbirds aim to be completely dry. Cormorants, for example, have outer feathers that partly soak up water, which reduces buoyancy and helps them dive after fish. That is why they are often seen standing with their wings spread out to dry, something ducks rarely need to do.

Water off a duck’s back

The English saying “like water off a duck’s back” describes criticism that has no effect on someone. It is a good description of real feathers: water landing on a healthy duck forms beads and runs off almost at once.

Next time you watch a mallard on a park pond, look for it preening and then shaking its body and wings, settling every feather back into place. That routine, repeated many times a day, is what keeps it dry. Waterproof plumage also works together with a clever heat-saving system in the legs, explained in why don’t ducks’ feet freeze?