Braxton Brigade
Songster
What are their symptoms?
We must get to the bottom of this, I would hate for you to lose the majority or even all the members of your flock.
Personally I would go ahead and treat everyone with Pour-on Ivermectin in case parasites (mites, lice, worms, ect.) are causing them to be so severely anemic that they die.
I find myself thinking that this (parasites) might be your only problem.
Mainly because the smaller chickens are going first, being smaller they would die of anemia much faster than the bigger guys/gals.
I would also as a precaution add some molasses and poultry vitamins (if you have some on hand) to their water so they can get a bit of a boost in the meantime.
You can buy nearly all of those things at tractor supply, co-op and maybe even another large named farm store.