You guys are going to bash me for this but that ok.
I hunted behind a friends GSP several times & though she was great. The only thing I did not like about his GSP was she tired out too fast. The dog had regular exercise in the off season. Could this problem be strickly diet related?

I hunt with a Golden Retriever. I know what you guys are going to say. (the golden retriever is not a bird dog.) I have trained my golden retriever to hunt Pheasent & Mountain Grouse. She can find the birds with her nose & retrieve them after I shoot them.