chicken with feathers gone on but?

I am not a chicken expert, but is she swollen back there?

How old is she?

Is she molting? Or it could be she has lice and the others are picking at them...

First check for lice/mites.

They are all molting right now but I have never seen that happen. And if she had lice or mites wouldn't the others have it too

Not always.

We has one that lost all it's tail feathers trying to get away from a opossum. She was so traumatized she died after a week.

I can't remember what animal it was but they eat the but out of their prey 1st. Make sure your pen is secure. The feathers should grow back just fine.

It could be either parasites, so I'd check for those first.

If you don't find any than it is probably the other chickens pecking at her. There are several reason why they'll do this - stress, boredom, or protein deficiency (they'll eat the feathers for that extra protein) - and once they get started it is hard to stop. Applying something like Blu-Kote will darken the skin, making the other chickens less likely to pick at it, and if that was the problem the feathers should eventually grow back.

It could be lice/mites, but It can also just be the other hens. Hens go on cannibalism when they're bored because they don't have enough space...

One reason I asked if she was swollen, is sometimes loosing the feathers can mean eggs are getting stuck inside their body. Unfortunately somewhat common in older heavy laying hens.

ThreeHavens said:

One reason I asked if she was swollen, is sometimes loosing the feathers can mean eggs are getting stuck inside their body. Unfortunately somewhat common in older heavy laying hens.

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True

I would clean and check her vent. Look for bugs. Look for signs of infected feather based. Also how's her poop?