The only way to know if its mareks is to have the bird necropsied. It looks like mareks. Sadly, I have it in my birds. Usually in the later stages they do get a weird green poo that has a stronger smell. In the beginning I did a lot of research and tried a lot of things. I could keep them going a while sometimes but the ending was always the same. I also found it unnecessarily put the other birds at risk because it is so contagious. Now, I just put them down ASAP, not only for the sake of the others, but also because its emotionally easier than putting so much into a bird then have to put it down.