Well you discovered your problem.
Most seeds are very low in protein and vegetative sources are always deficient in a couple essential amino acids.
Young and growing quail need somewhere around 25-27% protein. Adult breeders need between 18% and 20% protein.
A non-medicated turkey feed will do the trick.
Quail chicks natural diet is almost exclusively insects which is why the protein requirements are so high. Adults diet is about 80% seeds and grains.
Feathers are 93% protein and they are satisfying the need for protein by eating them.
Diet, space and parasites are the first places to look when you see this habit in any birds.
If you don't live where turkey or game bird feed is available, try increasing the egg or adding fish or insects to the diet.
Edited by ChickenCanoe - 5/17/16 at 4:45am