Sometimes birds will vary in shape and size, even though they are all eating the same food as each other. It's like my sister and I - we have the same parents, but she is tiny and I will put on weight if I so much as look at a biscuit sideways!
The main thing is that your chickens are eating, drinking, alert and active. Birds that are lethargic, isolating themselves, fluffed up, losing masses of feathers, lame, have runny noses and eyes or are experiencing respiratory issues are definitely those you should be concerned about.
That being said - I would look at the basics. Have you wormed everyone? Have they all been treated for lice and mites? Are they sleeping in a clean, dry, and well ventilated area? Do you provide adequate nutrition (growers pellets, grit and oyster shell)? Is there shelter for them when they venture outside, and do they have access to a spot where they can dust-bathe?
Once these matters have been attended to, I would spend some time observing the birds. It may be that some are just quieter in nature than others. Try and get to know them all, so that if they exhibit behaviour that is not 'normal' you will know that something is up. Then you can seek the advice of a vet if required.
I am only new to chickens myself but I have found that there is much to worry about if you let your mind run wild! Try and relax and enjoy your birds, and if anything happens that makes you think they are unwell, deal with it in due course. You (and hopefully, they) will be fine
Krista
I have literally been looking around all day for an organic solution like DE
