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There can be more than one reason why a hen would behave lethargically. When you see a hen acting droopy, you need to take her indoors and carefully inspect her from head to toe.
Start with the crop. Empty? Full? Sauerkraut smell? Hard lumps? Check in the morning first thing - if it's not empty, she has a crop disorder.
Check eyes and nares. Any swelling or mucous discharge? Bubbly eyes?
Check poop. Normal or runny? Color?
Now we're down to the far end of your chicken. Swelling in the abdomen? Do you feel something hard that could be an egg? Any discharge from the vent? How about lice or mites on the skin around the vent?
Legs and feet. Check bottoms and tops of feet for swelling and black scabs. Any other signs of injury? Scales - smooth or are any lifting up and separating from the legs?
Balance - is she able to stand? Or does she wobble and try to prop herself up on a wing to keep from falling over?
Do your chickens free range? Any machinery leaking fluids onto the ground where the chickens pick up grit and scratch for bugs? Insecticide use lately?
Somewhere in there are clues.