To the OP, since this post was revived I re-read the whole thing and a thought popped into my head... actually a question. How are you walking in to flush the bird? Walking along side the dog encourages creeping in because you have become competition. Same thing with another dog there... it's competition. Have you tried circling around and coming towards the dog (but not directly at it)?
Agree with possibly above.
2nd thing - is the dog doing this on pen birds? What about wild birds?
Dog could be doing it on pen birds as they will smell different and the dog knows the difference. My first dog never liked pen birds and would always act differently with them so I just never used them and only ran her on wild birds. Taking a wild guess, pun intended. :thumbsup: