Thank you for the information. Though not sure we have the answer. The bird's 'mate' does not 'mate' with it. In fact, it basically has nothing to do with it. There are other birds in the aviary, some weiros and a pink and grey gallah. The birds get fresh vegies such as tomatoes, peas, corn, broccoli, capsicum, lettuce as well as toast or bread.
The aviary is in the open with areas covered and protected from rain and wind, though often they sit on the swing in the rain. They eat sunflower seed and mixed budgie seed but don't have pellets. I have pellets when the weiros have young ones and give them to the mums to feed their babies and also for me to take over the hand rearing/taming.
I'll try the plastic sheeting, test for mites and get some pellets. What if there are mites? How do I get rid of them.? Though I am still wondering why only one bird is affected and losing it's coloured feathers.
These birds (the 28's) aren't the tame ones so it will be difficult to get them to a vet, but not to say I won't try. Thank you for your information.