I second the 870 Police Magnum, and if you don't mind the glossier finish, the Wingmaster, with a second barrel threaded for RemChokes.
The ext (extended magazine) tube, lets you load 2 (or 3? I don't use them) extra rounds for HD. For migratory birds, you still have to plug the magazine to only hold 2 rounds, I believe.
The chokes are screw-in pieces that go into the ends of barrels threaded for them. They constrict the end of the barrel to differing degrees, offering better performance depending on the size of the shot.
The second (hunting barrel) should be one threaded for RemChokes. Depending on what size shot you use for your bird hunting, I suspect a modified choke will work best, maybe a full. Best thing to do is get the gun and extra barrel, and as many different brands of your preferred shot size, and pattern them.