If the loss of down is in the category of "normal", you should be able to see tiny stubs of new feather growth pushing through the skin under the thinning down. These pin feathers will cause down to fall out as they grow in.
If you see large areas of naked skin without these new pin feathers pushing through, you need to figure out why. It could be parasites. Examine the skin closely in the area near the vent for tiny, moving red "dots", indicating lice. Watch for lots of scratching going on.
Watch the behavior of the chicks for signs of an individual plucking down from its neighbors' bodies. This is a not uncommon problem. You might be able to solve it by disciplining the culprit with a gentle poke on the back each time you see it nibbling on a mate.
Sometimes, though rarely, there will be an underlying pathogen at work. Watch for lethargy, watery poop, and other signs of sick chicks. It might also be a genetic defect.
I am feeding them wheat and corn and vegetables and sometimes meat or leftover.
I have 20 other chicks, they r about 40 days old i think and all others are full of feathers now exept that one.
he acts normal and eat and run regularly.
i will chexck for signs for growing feathers when i catch it, it is now playing somewhere and i cant see it.
Do you have Naked Necks (Turken) in your breeding flock? it's pretty rare but occasionally you get a chick that is pretty much naked all over, even more naked than that. Kassaundra has posted about making clothing for hers.
That's the only explanation I can think of. It is genetic. Since it is one chick and not all, it is not a flock problem. Whatever is going on is with that chick only.
Since he has no feathers I think he needs to be kept in the shade or have some type of shirt/sweater to keep him from getting sunburned. Depending on weather he will probably need something warm for winter.
There are totally naked chickens bred to be that way, so no feathers to pluck when processing them. I understand they must be kept shaded or they would sunburn very badly.
I have seen pictures of a totally naked parrot though I believe she originally had feathers and due to some condition she ended up totally naked. Of course she is a house pet and her many fans have sent in little coats and sweaters for her.
He sounds to be a very happy, healthy(??) little chick.
You may also want to post at " people with house chickens."
Was this chick normally feathered before, and lost a lot of feathers, or was he always sparsely feathered? I'm a bit concerned that you are not feeding a prepared chick starter. I know that there are many ways to manage a flock, but... a prepared starter provides all of the necessary nutrients in the right proportions.