If it is lice here is a plan that will ultimately get rid of all the lice on the birds. It can be time consuming but will be worth th ewhile inthe long run.
1. Search for lice.
2. Give the bird a long warm bath. (This should drown a lot of the lice and they will fall off.)
3. Dry the bird to a slightly damp state.
4. Dilute some horse fly spray. (I don't know what ratio there is of water to fly but dilute it to whatever you feel comfartable in applying to the bird.
5. Put it in a spray bottle and have another person hold the chicken on it's back. Search for lice and every time you see even just one, spray it. Spray the eggs also. Do this until you have sprayed down the entire bird and then flip it over to do it's back.
6. Put the bird in a sunny spot or wrap it in a towel ande let it dry. Do NOT rub the brid dry.
7. After the bird is dry, rinse it and then dry thouroughly.
8. Dust the bird.
I know that this sounds very over complicated but it reallky works. I had a bird that had a few lice and just dusted it and it didn't work. I thought it did and went on a ten day trip only to find the bird swarming with them. Their eggs were all over my hen. Then my mom thought to use diluted fly spray. I used this plan and every single louse was killed. The eggs didn't come off for a few months but never hatched. I was a little skeptical about using the flly spray myself but I tell you, this plan will eliminate even the toughest louse infestation!