You might want to get yourself a digital kitchen scale (see my avatar) and get a baseline weight on your birds. At their age, they should be gaining weight, so any loss is cause for concern.
The poo page has a few pictures labeled normal that I think are abnormal.
Below are normal, I agree. If the majority of a chicken's poop does *not* look like this, there is probably a problem.
For example, if one sees this all day long, I'd be willing to bet that the chicken has a...