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Even with the very longest lens you have to get very close to a small bird like a Downy. Even with a 600 you'll find you have to crop if you're much more than 20-25 feet away. Unless you have all the time and energy in the world, having birds come to you is preferable because you have better control over getting light at your back and having a good background. ( I can't emphasize how important background is for a good bird pic.) If you don't have bird feeders where you live try to find a place where Downys frequent and hang a suet feeder and maybe a small seed feeder. Set up a comfortable place from where you can wait and shoot. Of course you probably won't want a picture with the feeder but you'll find that they often land nearby at the same spot before feeding. Hope this helps a bit and good luck.