Feather loss:- When a chicken is about a year old she will start to lose her feathers, this is because she is in a moult. This is completely harmless process of plumage rejuvenation. Make sure she is well fed during this process as it takes lots of energy to grow new feathers and maybe supplement her diet with extra protein & vitamins. This will also cause her to stop laying until her feathers have grown back.
Mites:- Red mites - these mites are difficult to detect in the day as they live in the crevices of walls, perches and nesting boxes. It is only at night that they come out to feed on the chicken's blood. These parasites are pale grey in colour but turn bright red after feeding. These mites can be very serious to the chicken's health if left. To treat thoroughly clean out the housing with an appropriate red mite treatment.
Northern Fowl Mites - these mites are found on your chicken both during the day and night. To treat, use a mite powder/insecticide this will be the most effective, as the parasite cannot survive off the bird for very long periods of time.
Scaly leg mites - these mites burrow under the skin of chicken's feet & legs. The normally smooth scales become distorted and swollen with a white crusty material and the scales can fall off. These mites are highly infectious and a thorough clean of the chicken house is essential. To treat dip the chickens leg in surgical spirit twice a week or cover them in vaseline.
Lice:- Lice are found on the skin and feathers of the chicken, feeding on the blood, dry skin, scales and feather parts. Early detection is important as lice are an irritant to the chicken. Poultry lice are light brown and roughly about 3mm in length. To treat dust your chicken with lice powder..
Complete and guaranteed prevention of external parasites is unrealistic because they are transmitted via wild birds. Good hygiene and regular inspections of both house and chicken are the best way of limiting this problem.