I ended up taking my gouldian to the vet for a diagnosis as another of my (equally elderly) birds also developed a wheeze. Put them both in a hot box and treated them both with Ivermectin. After 4 days there was an improvement in both, but was really anxious to have a diagnosis. The vet agreed that air sac mites seemed the most likely cause and treated the gouldian for a secondary pneumonia. I have spent the weekend catching my other birds and totally stripping and cleaning their aviary, replacing all perches, brush etc after treating all the birds. Kept them all in cages for the weekend while this happened and noticed my other 2 gouldians have developed large pale droppings. Is this a side effect of the Ivermectin? Have tried to look it up with no success. My 2 old boys are now wheeze free and have graduated to a cage inside at night and outside under the pagola in the sun for the day time.