Like water off a duck's back

Feathers are one of the things that make a bird a bird. They look fabulous, but they have very important functionality, too. Of course, feathers enable most birds to fly. They also help with temperature regulation – something that’s especially appreciated by birds that live in difficult climates. Feathers provide camouflage for birds that need to stay on the down-low. And for other birds, feathers enable them to show potential mates what hotties they are – and what great genes they have.

For dabbling ducks and many other water birds, feathers also provide waterproofing. Dabbling ducks are the ones you might see with their butts sticking up in the air as they feed on vegetation and invertebrates in shallow water. They don’t dive for their dinner, but they do get wet. To stay afloat, they really need feathers that let water roll off, ahem, like water off a duck’s back! Diving birds like anhingas, on the other hand, need feathers that let them swim underwater. How do feathers meet these very different needs? Physics!

This black-bellied whistling duck has been dabbling for food below the surface of the water. Water is beading up on its back and neck thanks to water resistant feathers.

This black-bellied whistling duck has been dabbling for food below the surface of the water. Water is beading up on its back and neck thanks to water resistant feathers.

Feather Physics

Feathers are little engineering marvels. They are made of an insoluble protein called keratin. That’s the same stuff your hair and fingernails are made out of, but the type of keratin in bird feathers is a tougher version. Each feather has a central shaft. The portion of the shaft that attaches to the bird’s body is called the calamus. The portion away from the body is called the rachis. Coming off either side of the rachis are tight rows of fine branches called barbs. I’m sure you’ve picked up a feather before and noticed that these barbs seem to stick together, keeping the feather flat and giving it its overall shape. That’s because there are tiny little branchlets on each of the barbs, too. These are called barbules, and they have tiny little hooklets on them that hold the whole thing together in a nice, neat, tidy way. All of this interlocking allows the feather to function as a single unit rather than a whole bunch of individual filaments when air flows over them. That’s super important to the feather’s functionality for flight and thermoregulation.

Close-up of feathers on an anhinga's wing. The rachis and barbs are visible on each feather, but the barbules and hooklets are too small to be seen.

Close-up of feathers on an anhinga's wing. The rachis and barbs are visible on each feather, but the barbules and hooklets are too small to be seen.

If ducks and anhingas have the same basic feather design, how come one floats like a cork and the other one so easily swims underwater? They have applied the basic laws of physics to their feathers to make them either water resistant or fully wettable. (Okay, they probably aren’t consciously thinking about physics; evolution has done that work for them through natural selection.)

The barbs on ducks’ feathers have just the perfect spacing between them to trap air. Water cannot cling to air bubbles, so voilà! The feathers become water repellent, and water rolls off the duck’s back. Plus, those air bubbles trapped in the feathers provide insulation and add to the duck’s buoyancy like a whole bunch of teeny tiny life preservers.

Water rolls right off the back of this bathing duck.

Water rolls right off the back of this bathing duck.

Anhingas apply the laws of physics and their extensive knowledge about the physical properties of water to a different end. The barbs on their feathers are just a little bit closer together. This prevents air from clinging to the feathers, allowing water to penetrate the feathers instead. Having fully wettable feathers allows anhingas and other diving birds, such as cormorants and grebes, to swim underwater to catch their dinner.

This pied-billed grebe has been diving for food. You can see it is soaking wet. Water does not roll off this bird's back!

This pied-billed grebe has been diving for food. You can see it is soaking wet. Water does not roll off this bird's back!

Feather Care

Given how important feathers are to birds, they spend a lot of time taking care of them. That means preening. Lots and lots of preening. Birds preen by fluffing up their feathers, then using their bill to get those barbs and barbules nicely aligned and interlocked. This also helps get rid of any gunk or parasites they might have picked up. While they’re at it, they condition their feathers with a little preen oil – sort of like you might apply conditioner to your hair. This helps keep their feathers from getting dry and brittle.

Mottled duck reaching for preen oil.

Mottled duck reaching for preen oil.

Roseate spoonbills use preen oil, too.

Roseate spoonbills use preen oil, too.

Where, you must be asking, do birds get preen oil? No, it does not come in a bottle; it comes from the uropygial gland (also known as the preen gland) at the base of their tail. The composition of the waxy oil produced by this gland is complex, and varies from species to species. It may even have some helpful antimicrobial properties. While preening, birds just reach back there, get a little preen oil on their bill, and preen away. There are a few areas they can’t reach with their bill, such as the top of their head. To preen those feathers, they might scratch the area with their foot. (And all this time I just thought they had itchy heads!)

Preening advice from a mottled duck: After getting preen oil on your bill, preen any individual feathers that need straightening and oiling. Pay attention to your belly feathers, because you float on those!  To oil your head, rub it against oiled feathers on your back. If you've missed any spots on your head, finish preening by scratching with your foot. Now you're ready to go!

Preening advice from a mottled duck: After getting preen oil on your bill, preen any individual feathers that need straightening and oiling. Pay attention to your belly feathers, because you float on those! To oil your head, rub it against oiled feathers on your back. If you've missed any spots on your head, finish preening by scratching with your foot. Now you're ready to go!

Now let’s dispel a couple of myths. It was once thought that preen oil is what made feathers waterproof. While it might make a contribution, ornithologists now know it’s the spacing between the feathers’ barbs and barbules that is most important in keeping birds dry or wet. I still see a lot of publications that say anhingas are able to swim underwater because they don’t have oil on their feathers. Anhingas and other diving birds do, in fact, use preen oil on their feathers to help keep them in good condition. (See the photographic proof, below!)

Anhinga getting preen oil on her bill.

Anhinga getting preen oil on her bill.

Anhinga applying preen oil to a feather on her wing.

Anhinga applying preen oil to a feather on her wing.

It is also commonly said that, after swimming, anhingas need to dry their wings before they can fly, but it turns out that’s not true, either. If you watch enough anhingas, you’ll see they are perfectly capable of taking flight right from the water if they have to – although it takes some effort. When you see them on shore with their wings spread, it’s because they’re using them like solar panels to warm up.

When anhingas spread their wings to warm up, they almost always have their back to the sun to maximize the amount of heat their wings can absorb.

When anhingas spread their wings to warm up, they almost always have their back to the sun to maximize the amount of heat their wings can absorb.

Other adaptations

Feather physics isn’t the only thing that helps birds either float or sink. They have some other cool adaptations, as well. My favorite is that birds have a series of air sacs as part of their respiratory system. These sacs help birds breathe extra efficiently, and they also make ducks more buoyant in the water. Like most birds, ducks also have hollow bones to keep their weight down. Cavities in the bones are connected to the air sacs, so the bones can be filled with air for extra flotation. In other words, ducks are beautifully adapted to maximize buoyancy.

Adapted to float.

Adapted to float.

Not surprisingly, anhingas’ adaptations are a little different. They have air sacs, too, and they can be inflated or deflated as needed to regulate the degree of buoyancy. Their bones are denser than most birds,’ contributing to neutral buoyancy instead of flotation. And, if they are feeling a little too floaty, they drink some water to weight themselves down. If they need to stay at the surface, they can use their tail feathers and wings to keep from sinking. In other words, anhingas have a lot of control over how buoyant they are at any given time so they can hang out on the surface, then dive at a moment’s notice.

When anhingas swim near the surface while foraging for food, they are not very buoyant. All you can see is their head and neck. You can see how they got their nickname, "snake bird." Riding low like this enables the anhinga to easily slip underwater to spear a fish.

When anhingas swim near the surface while foraging for food, they are not very buoyant. All you can see is their head and neck. You can see how they got their nickname, "snake bird." Riding low like this enables the anhinga to easily slip underwater to spear a fish.

The next time you go for a swim, you might be sorry you don’t have feathers to help you float. But you should be happy you don’t have to preen thousands of feathers when you get out of the water!

By the way, if you don’t like this issue of Nature Notes, I’m just going to shrug off your complaints – like water off a duck’s back…


Fun Feather Facts

• Feathers contribute up to 15-20% of a bird’s total weight. A bird’s skeleton is usually only about half the weight of the feathers.

• Waterfowl and birds that live in cold climates tend to have more feathers than other birds, relatively speaking.

• Hummingbirds have about 1,000 feathers, while swans might have 25,000.

• Most birds have a uropygial gland, but some don’t, including ostriches, woodpeckers and pigeons.

• Some birds that don’t fly, such as ostriches, don’t have hooklets on their feathers to keep them neat and tidy and flat. As a result, these birds tend to look fluffy.

• When it comes to preening those hard-to-reach places, some birds help each other out in a practice called allopreening.

• Some birds come out of the egg naked, or with just a few downy feathers. These chicks are known as altricial. They rely on their parents to keep them warm until their feathers grow in. (By the way, humans are also altricial. We’re born naked, too!)


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