It's not impossible. The post with GS/Rott/Husky mix was able to get a dog that ran ahead or range ahead. The GS side of the dog certainly has the smarts for it but not as strong nose as a bird dog or hound. But it will still find birds. I've heard of a GS professionally trained for birds.

If I own a mix this is how I'd train it. First, get the dog hyped on birds. I'd go out to a game farm, buy a few birds, tie the bird legs to a long string and let the dog at it or pull some wing feathers so it won't fly far.

Next get a check cord and teach it to quarter.

Lastly, teach it to fetch.