I'm so sorry! I know how you must feel. A few years ago Marta dug up a nest of baby rabbits, and I tried to nurse them back and I fed them formula with an eye dropper, they were all doing really well, two of them looked healthy and even opened their eyes! But then they died mysteriously, I don't know why and I still feel very sad thinking about it, like I did something wrong.
It wasn't your fault about the baby bird- you did a good thing by protecting them. I hope the other baby is ok.