I really don't know about any of those things causing them to pluck because my birds never get any human food - well, that is not strictly true because Isis and Davy love to steal food from us and my husband has a soft spot for Isis so, whenever he is having his breakfast toast, she begs him for a piece and, unfortunately, she gets it!
But I will tell you that, in my personal experience, it's not a food allergy that makes them pluck. At least, if there is such a thing, I've never had a bird that has it. The pluckers I've had did it for psychological reasons or just plain force of habit after doing it for years and years - but even those get better. Linus only has the middle of his chest plucked now whereas, when he first came to me, you could see skin all over his body: back, neck, wings, chest, belly, sides, etc. He still barbers his primaries something terrible but it seems that he is beginning to allow more contours and down to remain so I haven't lost hope that he will, one day, look a bit more 'normal'
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