Donald's beak would seem to contain all kinds of densities allowing him to bend it in many ways. It can also be stiff and hard as a board.

Disney was not keen on their characters being drawn from below thus exposing their beaks from an unfavourable angle. Still, Barks did so on numerous occasions.

Donald's tongue is not necessarily rooted in his throat. It is frequently seen in other parts of his mouth, if it is not missing altogether.

Donald's beak changed length in proportion to the size of his head several times. In Scandinavia this has evolved into a serious form of dedicated research 'science' known as the Rostrilongitudequotient(!).

Barks would occasionally seem to have some difficulty drawing upper and lower beaks at the approximate same lengths. Many times Donald received a too short lower beak compared to his upper one.

Donald's beak easily opens in such a wide angle that Barks had to place the lower beak beside the neck in order to obtain the desired effect. This usually also meant that the lower beak was drawn shorter.

All the ducks have a solid rim along the front end of their beaks, a fact that Barks forgot on numerous occasions.

Characters wearing pince-nez have an astonishing ability; the eyeglasses sit firmly on their beaks at any angle.

The ducks have teeth but they are only visible when needed. Also,  their tongues are able to pass through clenched teeth.

The beaks can take unexpected shapes for short periods of time when their bearers get surprised, annoyed, or attacked. Examples are curling, sneering, and wrinkling.

Barks tried to give specific bird characters their expected beaks. But he did not always succeed; here is an owl character without a lower beak!!!

The beaks are highly flexible and can form into smooth shapes enabling them to deal with delicate matters such as playing the flute, whistling a tune - or kissing a lady...

It is commonly known that New Zealand's flightless national bird, the kiwi, lays some of the largest eggs in the world compared to its size!
But the bird is unparalleled in another field: Scientifically, the length of a bird's beak is measured from its nostrils to the tip of the beak. This can sometimes lead to astonishing results; the kiwi technically has the shortest beak in the world (despite of its rather considerable and noticeable length), because its nostrils sit right on the tip of its beak!