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I'm sorry you are having trouble with your chickens.
Can you tell me, how large is your coop?
Are they locked up every day, or are they getting out at all?
Do you have a rooster, or just hens?
Is there any loss of feathers on their backs or just their heads/necks?
Did you happen to open up the dead birds?
Okay, that helps. If you haven't noticed any "snick" or sneezing in your girls, I would encourage you to spend some time standing still for long enough in the coop, as this is a symptom that is easily missed and can indicate air sacculitis (very common in cold weather when hens are cooped up).
I would also recommend that you ask your local extension agent where to bring the dead birds for post-mortem necropsy so they can determine what caused the death (keep them cold until you get them there). This is a free service in most areas because of health concerns.
With no symptoms other than feather loss, which may be due to pecking at each other, I cannot tell you exactly what is going on. Most likely the scratches they are inflicting are getting infected and causing a dermatitis. This can cause mortality in the flock.
There aren't any diseases in chickens which cause feather loss of the head and death(in turkeys there are, but that's different).
My suggestion is to do two things: provide antibiotics (unless you are strictly organic) such as tetracycline (available at Tractor Supply Co), and enrich their environment to keep them from picking on each other. Give them extra green leafy veggies to pick at and eat, try spreading meal worms on the floor so they have to peck and scratch to eat them. Give them paper towel rolls to rip apart and other things they might like to tear. Keep them busy and they will peck less at eachother.
Also, adjust your lighting so that it is lower. 40 watts is enough to keep them laying, but low enough that they wont peck as much.
Hope that helps.
Where do you get your feed? The feed stores almost all sell the packets of tetracycline. Try there or any farm supply store.
What type of feed are you giving? What is the protein level? Any chance the feed has gotten wet?
I'm not as concerned about giving greens for the nutritional value, but to keep the girls busy. If they are not pecking at each other, I'm not sure I have the right answer for you. Other than scratching and dermatitis, feather loss can be nutritional or just molting (too young and wrong season).
One more thing to check...look carefully for mites. I'm guessing you are on top of that, but just thought I would mention that.
Try the antibiotic. Sometimes these cases of sudden death are respiratory related. If any other symptoms arise, please be sure to get back into contact.
The feed is Nutrena Layer Crumble. 16.0 crude protein. I thought of mites. Would I be able to see them? I've looked, but can't see any. Claire
If you look long enough, you'll see little specks moving on the skin. You'll have to part the feathers. They may even crawl up on your fingers, but don't worry, they wont stay on you.