With birds you have beaks and, in some cases, toothed edges (with swans, geese, etc.) The beak keeps the individual feathers in shape so they stay in perfect condition for flight. The oil gland above the tail helps waterproof them too, all that I understand.
However in dinosaurs who have feathers (and some had quite large feathers) - HOW did they keep their feather shafts in perfect condition? They were toothed jaws, which don't smooth individual feathers back to good condition no matter how you look at it. How did the dinosaurs keep their feathers 'right'? You can't do it with claws, they had no beaks, the tongue won't do it (feathers don't react like fur on a tongue).