Dan is correct, you don't need big glass to get good shots. A friend of mine has a blind in her back yard and she regularly sets up a dead tree/log not too far away from it and slathers peanut butter and bird seed in some of the holes of the tree and she can fill the frame with the woodpeckers that visit it using only a 200mm lens. Strategically fixing limbs onto or near birdfeeders are great in that they give the birds a place to perch before and after hitting the feeder which is a big plus in that you have a good idea of where to focus so that only small adjustments need to be made and you have a natural looking scene as opposed to a bird feeder in your shot, unless that is what you are going for.