If you have chickens, you have to handle them. Do you have a cat, or dog? Can you imagine taking care of them without handling them? Any kids? Can you take care of them without handling them? Grab that girl up, put on rubber gloves if you feel the need, and give her a close inspection. Whether you find anything on her or not, I strongly suggest that you give her a good dusting or spray with a miticide. But, you can't do that without handling her, and parting the feathers to see to it that the product gets down to her skin. I agree, DE is a waste of time if there are parasites to be dealt with. As for your question about "Why is only one bird affected?" Research has shown that 10% of the animals in a flock harbor 90% of the parasites. A bird that is a parasite magnet most likely has an underlying health issue. She would be #1 on my cull list.