There are several reason why chickens lose feathers. Regardless of the reason for a loss of feathers, increased protein is recommended, since feathers are predominantly comprised of protein, but depending on WHY exactly your hen is loosing her feathers, some other course of action might also be in order. So a bit of detective work helps to decide what exactly you're dealing with.
Here are some of the common reasons chickens (and ducks) lose their feathers:
The ducks especially seem to shed lots of small soft feathers when they preen after getting out of their pool, but chickens preen also. This is just a natural hygiene routine.
Preening is the process that birds use to distribute oils from the glands located at the base of their tails over their feathers and also to check for bugs and parasites. The preen oil helps waterproof the feathers to some extent and also enables the hens to convert vitamin D from the sun into a form their bodies can use.
perfectly natural. The feathers lost from preening could be considered the equivalent of hair we lose when we brush our hair.
Indication - There generally is no visible sign of feather loss from normal preening.
Course of Action - none.
Molting is the process birds go through, generally triggered by shorter days, in which they discard their old feathers and grow new ones in preparation for the winter. [read more about molting here]
Indication - A molt is fairly easy to identify by the pattern of feather loss. A molt always starts at the head and moves down the neck and body to end at the tail. Both sides of the hen will be fairly symmetrical as far as missing feathers. Also, the new growth pushes out the old feathers, so there won't actually be bald spots - the new quills will already be protruding from the skin.
Course of Action - Additional protein. Good sources of extra protein are meal worms, canned tuna or mackerel, scrambled eggs, chopped meat and other meat scraps, sunflower seeds.
Broody Hen - A broody hen is a hen who decides she wants to hatch some eggs. She will stop laying and you will find her sitting in a nesting box 24/7. [read more about broody hens here]
Indication - The feather loss pattern on a broody hen is unmistakable. Broodies pull out their breast feathers to provide a direct bond between the eggs they are sitting on and their skin.
Course of Action - Discourage the broody if she isn't sitting on fertile eggs. [read how to 'break' a broody here] Also feed additional protein. Good sources of extra protein are meal worms, canned tuna or mackerel, scrambled eggs, chopped meat and other meat scraps, sunflower seeds.
Pecking - Feather pecking by other hens happens for several reasons: boredom, living in too close quarters or a protein deficiency.
Course of Action - Separate the bully if you can identify her. Spray the pecked hen with Blu-Kote which is anti-septic/anti-pick spray.
Be sure your run is large enough for the number of hens you have. Ten square feet per hen is the MINIMUM you should allow, but larger is always better. Provide your hens with things to amuse them: logs to stand on, branches in the run to perch on, a hanging basket filled with greens, a hanging treat feeder, plastic berry boxes filled with cracked corn or sunflower seeds, etc.
Parasites - Generally, if you can take some preventative measures, external parasites should not be a problem. Adequate dust bath areas, garlic in the daily feed and food-grade diatomaceous earth in the coop and nesting boxes should prevent parasites for the most part. [read more about natural pest control here]
Course of Action - Soak the hen in a tub of warm water, salt, Dawn dish detergent and vinegar. The lice should float to the surface. Dust the hen with food-grade diatomaceous earth. Repeat several days in a row until the parasites are gone. Increase protein to aid in feather regrowth.
Mice and rats have been known to enter coops while chickens are sleeping and chew on their feathers for the protein or actually pluck feathers out to use as nest bedding.
Course of Action - Make sure your coop door is locked each night and check corners of the coop and nesting boxes for existing rodents. Cover any openings in the coop with 1/2" hardware cloth to prevent rodents from gaining access. Planting mint around the coop or sprinkling fresh or dried mint in the coop can also deter rodents.
Amorous roosters can be really hard on hens. When they mount a hen to mate, they dig their feet into the hen's back and grab the back of her head with their beak for balance.
Indication - Bare spots on the back of the head and back, could also be blood and raw skin.
Course of Action - Be sure your hen/rooster ratio is 10-12 hens per rooster to ensure that no one hen is getting too much 'attention'. Separate the rooster in extreme cases. Hen saddles can also help both to prevent further injury and to protect the area while the feathers grow back.
Any sign of feather loss on a hen should be immediate cause for further investigation, but not necessarily concern. Much of the loss is perfectly natural and beneficial. In any case, additional protein is the recommended course of action to help regrow new, healthy feathers, but just a bit of further investigation can help you get to the root cause of the feather loss to better treat, or possibly prevent it.