http://edis.ifas.ufl.edu/ps044
diagnosis charts at bottom
(one more for you)
Please note that I don't know what is wrong with your flock- just trying to help and hopefully someone with experience in this can nail it for you!
You might consider worms if they are skinny.
Also mites/lice.
Look up lymphoid leukosis.
http://www.thepoultrysite.com/publications/6/diseases-of-poultry/202/lymphoid-leukosis
http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/203603.htm
Sorry I can't help more!