The Remington 870 is certainly popular, but the Mossberg 500 is, in my experience, just as reliable and less expensive.
GET A 20-GAUGE!!!
Less recoil, and you won't chew up a bird with shot.
Run 7 1/2 or 8 shot, use a modified or Improved Cylinder choke tube and you're in business.
I prefer double barrel shotguns, but now we''re talking expensive unless you opt for one of the very well made Turkish guns imported under the Charles Daly brand by KBI. I've tested a couple of these guns and they are first class, without the first class pricetag, and you find yourself using less ammunition!
Good luck!