Urgent--very sick white leg born, upside down black mites?
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I would get some permethrin right away and dust her or spray her for possible mites. Then repeat this at least once in 7 days. In the meantime, look over your other chickens for the bugs, and begin treatment. The bedding in the coop, nest boxes need to be replaced with fresh bedding, and those along with the roosts should be sprayed with permethrin as well. Here are some links to help you identify mites and lice on your chickens:
She could be anemic if the bugs are mites. Poultry Nutri-drench or Poultry Cell vitamins 3 ml daily would be good to treat her with for a week or so. Give her some egg, plain yogurt or probiotics, along with her usual feed. If she has not been wormed recently, fenbendazole or Valbazen are excellent wormers. Hopefully, she is just weak, but if she continues to be ill, just keep posting here.