I cannot stress how important it is to make sure the bird is getting enough raw produce which is the ONLY thing that provides them with the necessary 'food' for their gut bacteria! It's not only a matter of digestion, it's a matter of keeping the immune system healthy as, at least, 70% of the immune system is in the intestines.
This is about how evolution and the natural habitat worked to match the gut bacteria to each species:
https://www.sciencedaily.com/releases/2 ... 101440.htm
The next two are about the relationship between gut bacteria and the immune system:
https://medicalxpress.com/news/2018-03- ... mmune.html
https://medium.com/@thryve/the-gut-micr ... a85b9f3c99
This is about how the right gut bacteria is a natural anti-anxiety cure making a good argument about plucking birds not getting a good diet:
https://www.sciencedaily.com/releases/2 ... 190110.htm