What you describe is common in dogs and most do it to some degree but it often passes unnoticed .Your dog is evidently one for the bigger gesture! (At least yours is not doing the full pointer act. A pointer stands rigid, in a pose, with it nose 'pointing' and its body stretched forward and its tail out straight, pointing to where it senses prey, the bird, is)
The dog makes something of a 'question mark' with its front leg when its asking itself a question! If its puzzled , asking itself 'What was that ? What's happening? What to do?" it stops and raises one paw , slightly crooked, just as though it was trying to form a question mark with its paw.