Feather loss over the head indicates an outside factor, usually the mate, is involved. Young bird in the nest with feathers missing from their backs and head are being plucked by their mother - generally she wants them to leave the nest so she can lay again. Careful examination of feathers and skin will show lice or mite if they are present. Holding a feather up to the light will show feather lice very clearly - there can be massive numbers present in some birds. Budgies can have large numbers at the base of the tail feathers, so many that they look like mud stuck to the feathers.
Ail contains a water based insecticide that is harmless to birds but deadly to insects.
The long activity time of AIL means that adult insects, nymphs and eggs are all killed. The residual activity of this new insecticide will leave your birds insect free for many weeks. A.I.L.'s safety allows you to spray the birds themselves, cages, perches and anything else in their environment in the one operation. Being water based, AIL will not sting the birds eyes allowing birds to be sprayed all over while in their cage. Once you are happy that the bird is insect free, the next step in sorting out a plucker is to look at the diet.