I don't think there's any way to increase the amount of feather that she grows, but a lot of people swear by MTG to promote the growth of the feathers she already has.
I have a gypsy, so I'm an involuntary pro at the maintenance of feathers:roll:.
My advice would be to start the coronet band and pull all the feathers up and comb them from underneath, moving your way up the leg. This helps to keep them from getting matted and lets air get to them, and makes them look nice and poofy along the way
Try to keep them dry as well, when they get wet they can get scratches or the hair can dry out and break off. Hope I helped a little bit