Last August, my sun conure got extremely sick. Vomiting, lethargy, I would have lost him had I not taken him to the vet right away (I was lucky this happened on a Saturday). I am not sure as to what caused it but narrowed it down to 3 things:
1. I had a very bad flu (sneezing, coughing, runny nose, phlegm) and Boomer was always on me. Birds can't get sick from human virus but they can get sick from the bacteria that becomes abundant on us while sick.
2. He licked a yucky part of the shower door track. Don't judge haha, I keep his cage and immediate surroundings immaculate. Life caught up to me at the time.
3. The night before, I fed him boiled egg where the yellow part was not crumbly dry but gelatinous. I also bought the egg shamefully from a 99c store because it was the only close store that was open late at the time.
I have a strong feeling it was the egg. I was so traumatized by this that I have not given him any egg since. I know I should, and in time I will, but for now I'm giving him his calcium and animal proteins from other sources. I am sure it is safe though. Just make sure to buy the egg from a reputable source and to boil it so it is 100% cooked.