Hi Jane,
I'm very happy to learn that your Amazon has a top-notch avian vet. Hopefully he did a full workup with swabs and blood work.
Some feather discoloration can be caused by fungal growth. However, if you called this problem to the vet's attention, this should have been ruled out as a possibility. If it was rules out, then I think you may very well be right on track with the idea that your facial moisturizer or a product you use on your skin is the culprit. This would be especially true if the main discoloration is limited to areas which you touch or where you pet her. That is why I wanted to know if the distribution was even all over her feathers.
If there is discoloration in areas which you do not touch, however, then I think it would be related to something she ingests which is the culprit if fungus is known not to be at fault. If she has this same discoloration in areas she does not allow you to touch or where you do not usually contact her (her vent area, for example), then the cause is not your skin products.
In order to ensure your Lilac Crown Amazon lives the happiest and healthiest life possible, she needs to have a little change in her diet. Ideally, 35% or so of the diet should consist of pellets, seeds, or zupreem. The rest of her diet should consist of fresh fruits and vegetables, dark leafy greens (but very little spinach), and other people foods. Basically, you are feeding her the right things, but you simply need to increase the fruits, veggies, and other items quite a bit and reduce the Zupreem. This will help her get a balance of nutrients that will optimize her health. Click here for more information on what to foods to provide and which can be deadly.
I can tell you truly love your parrot and have put lots of time and effort into providing her with a good life. I do hope this information helps and that you can get to the root of the problem. If I can help further, please use "reply" after accepting this answer.
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