Chicken Walking on its Knuckles
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Curled under toes can be a sign of riboflavin deficiency. I would make sure that your feed is fresh dated, or get a new bag. Start some poultry vitamins in their water containing vitamin B2 (riboflavin.) If you have B Complex for humans, you could also use that. The sooner it is treated, the more likely it can be cured. Here is an article from The Poultry Site:
432.433.The hypovitaminosis B2 (riboflavin deficiency) is characterized with impaired oxidation processes and dystrophic changes in the peripheral nerves. The typical clinical sign are curled toes due to paralysis. In the beginning, the toes are slightly flexed and chickens tend to stand on their hocks. In moderate cases, a marked leg weakness and toe flexion in a different extent are observed.
434.In severe cases, the toes are completely curled downward and inward and complete weakness of legs is present. A considerable improvement and relief could be expected if the treatment is initiated in the initial stage of the disease. Water soluble vitamins, that are easily utilized, are recommended.