The advice hub The Depluming Mite
The depluming mite (Knemidocoptes gallinae) is related to the scaly leg mite and is also a burrowing mite. However, whilst the scaly leg mite burrows in between the scales of the leg, the depluming mite burrows into the feather shafts particularly on the head, neck, back, belly and upper legs.
This burrowing causes damage to the tissue which oozes with a nutrient rich fluid on which the depluming mites feed. This burrowing causes irritation and pain to the chicken causing it to scratch and to pull out its own feathers. Severely burdened chickens will lose weight and will lay fewer eggs.
One interesting fact about depluming mites is that rather than laying eggs, they give birth to live young and they can complete their lifecycle in as little as 17 days. They tend to be most prevalent in spring and summer, with numbers falling in autumn. The mites can only be transmitted between birds by direct contact.
Treatment for depluming mite involves using an appropriate product. Treat the house as you would for red mite infestation.