That didn't last long!

[SquishSquash]

My baby bird is down again. Her crop is squishy and she's huddled off by herself. Should I try to nurse her back again, or do I consider that she may just be a weakling. I would really like for her to be well and part of the flock but I know I also have to be realistic AND protect everyone else. I don't want to breed weakness, and I don't want her to suffer like this on a regular basis. If this is sour crop... how do I cure it in a 4 week old bird? I appreciate any thoughts and help! p.s. All other babies are fine and all the living conditions are the same.