The answer depends on your expected shots. Are you in a situation where the birds will come in and land on a regular basis, giving you 25-30 yard shots? Or are they flying by slightly lower than the 747's and you are hoping for a 40-50 yard shot?
Alot depends on the expected distance you think you will be taking your shots.
Modified is a good all around choke for steel shot. Where I hunt you get a mix of close and long shots and modified works well. Some guys with O/U's and double guns use imp cyl and modified.
Always pattern your ammo since there can be great differences in gun and ammo.My Benelli gives a pattern like a turkey choke with BB steel shot and modified choke !! And learn the effective range which for BB steel is 25-45 yds.
Unless you pattern, you're shooting blind. Pick a load for the species, pattern at 25, 35, and 45 yards to see what happens with a given choke and go from there.
I'd try Light Modified for starters, and end at Full.