If you want to see something really cool, look up How to train your dragon Live spectacular. Its so cool, I get to go see it in a week too! This is really cool, and I think a nadder would look like this in real life, you know not all animated.
I have some sketches where I went full out with the parrot...ness (?) and....its just....didn't work for me...like....it didn't look right, and it certainly wasn't dragon-enough for my liking, so I did this..
His face/beak should be rounder though to keep in line with the avian appearance... Like I said, im not quite happy with the way this looks (which is sad because Nadders are my second favorite because they look like parrots)
Nadders are literally my favorite for obvious reasons haha.
Uhh maybe sorta basing it on a terror bird? Only make the beak more compressed and less elongated like the hammers they have. Like a sort of hybridization of a terror bird skull and a parrot? And then just give it scales and spines instead of feathers.