BODY

Although Barks' ducks at a glance look a lot like real ducks in body shape there is a clear difference; Barks' ducks are considerably more pear-shaped! They have a very small upper body and a very large lower body.
The proportion of their bodies as opposed to their heads are also 'non-duckish' in Barks' drawings: From the very start Barks distanced his Donald from the animated shorts' Donald by making his proportions 3½ heads tall whereas he was 4 heads tall in the films.
The bodies are also extremely tough; just think of the many times Donald has fallen from high places or been caught in severe explosions without getting seriously hurt...

FEATHERS

The ducks' feathers have the ability to change colour in different situations; cold water turn them blue, heavy smoking turn them green, rage and spanking turn them red. NB.: These specific colour changes are, of course, just qualified assumptions, because Barks always drew in black and white, but his occasional use of added pen strokes (as seen in one of the examples above) clearly indicate that the feathers did change colour.

None of Barks' ducks wear trousers or skirts, but it seems as if the females have some kind of frilly feathers around their hips that resemble the lacy ruffles of old-fashioned bloomers.

SKIN

What colour is the ducks' skin? We can safely assume that it is light, possibly pinkish, from studying Barks' black and white drawings in which he did not indicate any colouring. Examples: In WDCS230 Black Wednesday and in WDCS234 Riding the Pony Express Donald gets scalped, and we can clearly see that his skin is fair.

It is also plausible that the ducks' skin is humanlike; bruises, swellings, bee-stings, and black-eyes are telling examples.

Furthermore, in WDCS265 Raven Mad Magica de Spell discloses her wish - and ability - to get sunburned!

PLASTICITY

It is plausible that Donald's torso has the highest grade of plasticity among the ducks. At least he is the one that most often has been faced with deformations, but, usually, he just bounces back into shape as a rubber ball.
There are several degrees of temporary mutilations ranging from the lightest ones such as being rolled over by a barrel when he has been on the brink of drowning to the most severe ones where he is more or less reduced to a flat spot on the floor...