Big Uncle

You need a different barrel on your 870 to bird hunt.

You could probably trade your full rifled barrel to someone for a smooth bore barrel that would accept different choke tubes. Another route is to trade entire shotguns.

I see a bunch of used 12 gauge 870's with barrels that accept Rem Chokes for sale frequently at good prices. The 20 gauge seems a bit harder to find, but neither of them demand high prices.

Best of luck.