OK so if you dry the fluff behind that tissue you should see the bum, I am inclined to think it's the yolk sac. Check to see if you can see that bum. If you need to pick up one of the other chicks to see what it looks like do so. If you see the bum then its likely the yolk sac.I can. Just gave it food and water. Still quite lively and strong. I do not know if this is umbilical or butt or how to tell.
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Deep breath, you can do this, one way of the other.