Several good suggestions have been made. If it were me and I had the choice to do so, I would go with an 870 (or similar) with screw in chokes and # 6 shot. I know that others have suggested # 4 but # 6 will give you more pellets and still have the knock down power you need for the bigger birds. As others have said, tighter choke gives you more pellets in a smaller space for longer range. I would go with at least a modified choke, or maybe even full, depending on the distance from your gun barrel to the birds.