If you have the means, I would install a web cam or other surveillance camera to see what is going on. If it's happening at night, you might need to install a light too.
How many chickens do you have and what are their ages?
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Give us the following information. The more you tell us, the better we will be able to help you.
1) What type of bird , age and weight.
2) What is the behavior, exactly.
3) How long has the bird been exhibiting symptoms?
4) Are other birds exhibiting the same...