hi, we too have got ex battery hens and i have had one keel over from illness. oakey as she was called went from being friendly chicken, always up close, happy to be picked up, to one that any time you tried to grab hold of her she would run a mile. she started being the first one to go to bed, then one morning was collapsed on the coop floor. i lifted her out, she took a few steps then sort of fell over. i rushed her straight to the vets and he said she was painfully thin. surprising as she hadnt stopped eating (i know this form watchign her head constantly stuck in the food bowls) but not being able to get my hands on her for a good couple of weeks before there was no way to tell.
the vet gave her antibiotics and told me to keep her indooors for a couple of days, but she died in the night.
the vet also told me that due to the amount of rubbish they pump into them they can die just through their internal organs not being able to cope and that kidney failure in them wasnt uncommen. all that said i woudlnt expect this to affect the others.
please let us know what the vet says and i hope the rest of the brood are doign well.