Please don't be hard on yourself. It's entirely possible that you had nothing to do with this and it's just the natural progression of whatever is going on with her. I have worked really hard on trying to save some of my birds only to find out at necropsy that it was really out of my hands, nothing I did was going to make a difference. But I had to try. It's really hard sometimes to know exactly what's going on. I've had birds live that long, and you get very, very attached. When they begin to fail, I just make them as comfortable as I can, that can mean different things for different birds. Some I leave with the flock because I know that is where they are happy, some I isolate, so they can have calm and quiet. Depending on what is going on I decide whether to let things happen naturally, or whether to euthanize to prevent suffering. There is no one right answer. I know it's really hard, and you will miss her. She was loved, and well cared for, she made it to 13 which is really, really good for a chicken. You should not feel guilty.
