When I was reading about onions and birds, the only article I found that had any real information said that there hadn't been any actual research on onions and birds, but to not feed them just in case. It said that with dogs and cats (where the research has been done) long term exposure can cause hemaltyic anemia, which can be serious.
I wouldn't worry about one exposure too much though. I gave Puck some salsa with onions in it before I read about it, and he was fine. Now I just pick the onions out of stuff I give him, just to be on the safe side.
Somebody correct me if I'm wrong! This was the only factual article I could find!