FLUIDS

The ducks carry the same bodily fluids as humans do. They can break out in tears and they can sweat all over their bodies (even from their web feet!).

Donald seems to contain a lot of water which becomes visible on rare occasions; in WDCS109 'The Water Witching Stick' we experienced him with a badly swollen knee that turned out to contain water, and in WDCS207 'The Wild Burro Contest' he got so angry that steam emerged from his nostrils!

Although we have never seen the ducks' blood (and for a good reason: characters in funny animal stories do not bleed) we can safely deduct that it is red. Several blushings, angry red eyeballs, and spanked rumps are dead giveaways...

AILMENTS

The ducks are prone to illnesses caused by many outside factors and influences: They get fevers, colds, and measles just like us.

Their stomachs are not as galvanized as one might suspect; the ducks can succumb to bad stomach aches from eating bad food just as Donald did in WDCS0223 Lost in the Andes when he ate an old square egg.

Learn more about the ducks' ailments HERE.

GRAVITY

The duck universe seems to have a gravity of its own. In all the stories there are frequent examples showing the characters running high above the surface. Furthermore, when a character is thoroughly surprised his hat mysteriously pops far up in the air only to automatically re-position itself in the next panel. And when a character gets very angry he can literally hit the ceiling as Scrooge did in U$23 The Strange Shipwrecks!

The male ducks also seem to have another hidden ability; when angry or otherwise inclined they are able to lean heavily forward without gravity getting hold of them. Maybe they have some hidden, automatic suction device under their feet?

HEIGHT

How tall is the average Barks duck? Well, Donald once found himself surrounded by - presumably human sized - athletes at a sporting event. He seems to reach up to their hips in height (WDCS188 'Olympic Tryouts').
Also, Magica de Spell has measured Uncle Scrooge to be 3 feet tall (U$43 For Old Dime's Sake). In Barks' painting July 4th in Duckburg (see HERE) the ducks also seem to be about 3 feet (almost 1 meter) tall compared to the real humans.

WEIGHT

Barks never divulged the ducks' weight precisely. Presumably, Donald weighs in at around 90 pounds (circa 41 kilograms). At least, in MOC04 Maharajah Donald he was just 2 pounds short of reaching 100 pounds after his feathers had been filled with gold dust.

Once we witnessed Scrooge being weighed. This happened in U$09 The Tuckered Tiger in which he stood to loose his weight in diamonds. But in order to earn on his coming loss he filled himself with dentist's gas which in turn caused him to weigh 60 pounds less than nothing!!!

VOICES

How on Earth can it be possible to determine what the ducks' voices sound like from reading a Barks story??? The only authentic duck voices we do know come from the numerous animated shorts in which f.i. Donald's voice is reduced to being an undetermined squaking that mostly sounds like gibberish.
But that cannot be the case in Barks' duck universe! In the cartoons Donald's dialogue was rarely significant, but in the comic book stories it is essential to the plots, because we get most of our information from the spoken words. Therefore the ducks have to be speaking in clear and understandable sentences.

Numerous times we have witnessed Scrooge speak in foreign tongues and Donald and the nephews singing Christmas carols, and in both instances they have been understood without any difficulty.

Barks did hint a few times that Scrooge has a Scottish accent, but in one of his last stories, U$71 King Scrooge the First (manuscript and sketches only, art by Tony Strobl), we finally get a genuine statement from Barks' pen:
A fortuneteller hears Scrooge talking, and then he ponders: That voice! I've heard it before! The exact inflection! The unmistakable whirr of vowels grinding the edges off consonants!
Well, there you are...