The duck beak has a distinct appearance; it is relatively long, has nostrils, and it is rounded in the front, which is also furnished with a rim. In comparison, Gladstone Gander's beak has no rim, Gus Goose's beak is more pointy, and Gyro Gearloose's beak is that of a meat- or nut eater - pointy and sharp. Many of the ducks' beaks changed length in proportion to the size of their heads several times - especially Donald's beak (the only primary characters who did not have their beaks' changed at any time were the nephews).
Barks was not aware that he slowly changed Donald's beak, until his attention was drawn to it by a woman who was living nearby, and who was employed in Disney's cartoon colouring department. She met Barks one day in the street and told him that the current cartoon beaks were much shorter than those Barks drew in his comic books. Then he adjusted the beak to mainly follow that of the animated shorts' Donald at the given time. In a 1966 letter to a friend Barks explained: I did deliberately shorten the duck's beak around 1949 when my slow wits finally awoke to the fact that I'd been drawing them too long. Still, he always said that he liked the long beak the better. You can see how Donald's beak (as well as his whole appearance) changed over the years HERE.
Through time Barks demonstrated in endless ways that the duck beak was extremely versatile and flexible and able to change appearance in order to underline a certain mood or a specific feeling. In the rightmost example it has an ability to curdle up in a sneer.

The ducks' beaks are very similar in appearance to those of real ducks, but they are considerably more versatile. Although they appear to be more stiff than rubbery the characters' beaks have a clear plasticity when needed. They can fold, stretch and even be rounded for a kiss or to whistle a tune. They can wrinkle in a snear. And, apparently, Daisy can use her's to speedtalk...
When Donald is very surprised, enraged or commanding, Barks would sometimes place his beak at an impossible angle where Donald's lower beak seems to be far to the left of his neck.

What is inside the beaks? Amazingly, they seem to have teeth but only sometimes. As Barks 'explained' it: The ducks had teeth whenever they needed them! Still, we know that all the ducks must have a full set of them inside their beaks, because we have seen them gritting their teeth on numerous occasions. Still, they are very well hidden and not visible when they open their mouths. Another telling proof of the presence of teeth is that the ducks always remember to pack their toothbrushes when going on a trip!
The beaks also contain tongues. Very often they seem to be 'loose' ones depending on what the ducks are doing. Mostly the tongues start back in the throat, but quite often they start in the middle of the beak for no apparent reason. Furthermore the tongues are highly metamorphic; sometimes they are short, sometimes they are long, and they change from being oval to being triangular in shape.
A surprising detail is that Donald's beak occasionally contains dragon-like fire-breathing abilities, but only when he is highly infuriated! In calmer situations he just lets out steam - literally...

Donald's beak has a number of mysterious abilities and disabilities. Here are two examples: We all know that the ducks sometimes are shown with teeth or very long tongues inside their beaks. In the first example Donald has both, which makes you wonder how his tongue can penetrate his clenched teeth! In the second example it is highly curious that a 'duck-to-water' character such as Donald needs a diving mask to protect his beak - as well as extra fins for his web feet...
But the most intriguing and inexplicable peculiarity, however, derives from Scrooge's (and Gyro's) use of their pince-nezs. How do they keep them from tumbling down their very smooth beaks???

As we have learned the ducks' beaks did change length over time, but this is nothing compared to what happened to Daisy's three nieces April, May, and June over the years! Beak-wise they started as ducks but they then underwent some strange sort of metamorphosis; they lost the rim on their beaks which also became more pointy indicating that the girls ended up being another breed of bird.