Why do ducks, swans, and other birds that spend much time on the water float so high? I guess that more than 50% of their bodies are above the surface of the water.

I understand that flighted birds have much less dense bones than large land animals. Surely though, they are still mostly water?

My best guess is that they can trap a lot of air between their waterproof feathers and thie bodies. Is that roughly right?

Birds are very light, as you note, their bones are almost hollow.

When an object is submerged in water, it submerges to a depth where the mass of the displaced water equals the mass of the object*.

Hence a light bird (and they really are very light) will displace a very small mount of water and so not sink very much at all.  Hence so much of the bird remains above water.

Also, again as you note, the feathers play an important role in your perception of how submerged a bird is.  The submerged part will have feathers pressed down against the body, reducing apparent size, whilst feathers above the water will be at least partially away from the body, increasing apparent size, and thus distorting the proportions of submerged and non-submerged bird.

*edited for accuracy.  Alternatively, James writes:
The upthrust from the liquid depends on the volume displaced - if it's greater than the total mass of the body, then the body will float (mass of displaced volume of liquid = mass of floating body), otherwise it will sink (mass of displaced volume of liquid < mass of immersed body).

Last edited by Peter Falkingham (30th Jan 2008 12:03:03)

Still Peter, I expect a bird like a duck will trap a lot of air in its feathers under the surface, which will increase its buoyancy. For a nice counter example, look at the snake birds of South America. These swim along underwater but with the long neck and head above the surface giving them a Loch Ness Monster appearance and hence, the name.

Just to add to Peter's comment, different groups of birds swim at different heights in the water. The Gavidae and Podicipedae (divers and grebes) swim (on the surface) with more of their body submerged than do the Anatidae (ducks) and Laridae (gulls), this may be to do with their greater adaptation to a watery existence, with their legs far back on their bodies. Part of the 'jizz' for identifying water birds is the position they take in the water. Birds like the Anahinga swims with all of it's body submerged with only it's neck breaking the surface of the water. It can only keep this up for a short time (when it is hunting fish underwater) as, if memory serves, it has either no or a very poor preen gland for coating it's feathers with a nice oily, waterproof coating, leading to it become waterlogged, where it has to clamber out and bask in the sun to dry. By the by, this doesn't seem to apply to the Phalacrocoricidae (cormorants and shags) which are commonly reputed to dry in the sun (with their classic crucifix pose), rather this is thought to help aid digestion.


I'd just like to add a little more here.

The main factor to affecting water-birds is buoyancy when in the water and weight when in the air.

Ducks displace little water since their body is less dense than water. Their bones are hollow and connected to their respiratory system, so they are also full of air. More importantly perhaps, ducks are fatty birds. Fat is less dense than water, so the high proportion of fat in a duck will keep it's density lower than the water. Trapped air in the plumage also contributes substantially to the pressure difference that determines an object's position in the water.

Less fatty birds are denser and will sit lower in the water. Many ducks dabble, so they don't fully submerge, rather they up-end. Diving ducks are less fatty and tend to be smaller, or they have feet near the back of their body to kick their way down deeper.

Birds that swim underwater need to be more dense so that they use less energy to keep themselves underwater. Of course higher density generally means greater weight, which is bad for flying unless the weight can be lost. Cormorants and darters have very permeable plumage so they can increase their density by getting soaked with water - this can then be shaken off before flying. Other diving birds have let their ability to fly be reduced, penguins being an extreme example of this. Other diving birds use gravity to help by diving from a height, like kingfishers, gannets and terns - but these birds are not actually very good at catching prey if they miss the first strike.