Eddie,
The Labrador Duck had a fleshy protuberance on the lower distal (front) portion of the bill. The belief was that these were used in eating mollusks, clams and such. I have lots of information including many pictures of the bills on the remaining taxidermic specimens showing these fleshy parts. I took a little artistic license, as I'm fond of doing, and made a little interpretation. As for the paint its all oils.
Regarding the shape and the size relation in regards to the head I used Audubon's notes from his painting done in 1833 that my wife was able to obtain for me from the University of Pittsburgh Library wherein he described the size of the bill as nearly as long as the head. That gave the basis to start with. And I ran with that.