A 2X tele extender will fill the frame and at the same time reduce the effective f stop and resolution. Combined with a 300mm lens, it is your solution for wildlife unless you want to spend a lot for a 500 to 1000mm lens that you might not use all that frequently. When I was in the USAF in Alaska I had a cheap 400 mm (f/6.3 I think). I made moose "portraits" at lunch time at distances of 20-30 feet. The cows are about as smart as a domestic cow and pay little attention to people. Canadian geese were photographed at distances of 50 yards or so with good success. Of course they are enormous birds. I think for any serious wildlife work you will need an effective focal length of 500mm or longer. Mirror optics are available as well at bargain prices.