My fowls are light, not heavy compare to their size. What happen?

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I am back now that my flocks are having issues of being light.

Of cause, I don't know either is from the food they are eating? Or is from a particular disease.

But I will be so much happy if you can tell me what to do
Thanks

 

[casportpony]

What breeds?
What weight?
What are you feeding?

 

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Plymouth rock
1kg
Normal food