The ideal type of background for birds is either a blue sky or foliage that is so out of focus there is no definition in it at all. That's what you see behind this carmen bee eater I photographed in Ethiopia. I used a 600mm lens plus a 1.4x teleconverter, giving me 840mm of focal length. The background grasses are completely out of focus for two reasons. First, I used a long lens. Second, the background was far enough away from the bird that it became essentially a haze of color. Had the background been, say, two feet behind the bee eater, no matter how long my telephoto was, it would not be out of focus enough to make an attractive picture.