Poop question with picture
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How old are they?
Could be a worm, something long and stringy they ate or intestinal shed (but that is usually red colored).
General worming info is: Valbazen 1/2 ml orally and then repeated in 10 days for each chicken or Fenbendazole (Safeguard, Panacur) at 1/4 ml per pound for 5 days in a row.
More information about worming, treatment and withdrawal periods: