This is quite a common one I am asked when coaching, but it really depends on how steeply the bird is climbing, however as a rule try this:-
Always bring the gun up from underneath the bird and just sit the bird on the barrels as if you were trying to shoot its feet off. As 60% of your shot flys above your barrels you should bring the bird down with out a problem. If the bird starts to fly left then aim the same but at his left foot. or the opposite for the right.
Good luck and let us know how you get on