I have a 3 month old Australorp that is lethargic, has a droopy tail, and isn't eating. We have isolated her and she has access to food and water.
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Get her to drink some chicken vitamins with electrolytes in her water. Most feed stores sell SaveAChick, Durvet, or Poultry NutriDrench. At that age, coccidiosis as well a s dehydration could be a problem. Symptoms of coccidiosis are lethargy, hunched posture, not eating, and diarrhea with mucus or blood in droppings. Corid or amprollium in the water is the best treatment for 5 days.