My advice is to pay attention to the background. By changing your angle relative to the horizon, you can pick what the blurred background is made of, especially with a telephoto lens as the ange is very small. Try to place dark objects behind the bird to make them stand out more. If you are shooting up, get on a ladder and wait. I tried this last year with some humming birds who liked the top branches of a short tree, and that improved my pictures because it eliminated the bright sky behind them. I need to set up some feeders and perches myself to improve the bird-ability of my backyard.