My gun doesn't pattern too dense, I'm not a good shot, and I like my birds close. The only time I really misjudge is in a wide open field which I don't usually end up getting a shot in a field anyway. In the woods it's not hard to guess under 40 yards. If you don't use a rangefinder (I don't) and are not sure what 20-40 yards looks like then practice by stepping off the distance from a few different trees and look at the surroundings and try to get a feel for what the distance looks like using your depth perception. Not sure if that makes any sense, but guessing range is really just getting a feel for it more than actually trying to calculate anything.
Just remember if you think it might be too far, or your just not sure, then don't shoot. When that bird is close enough you will be 100% confident he is close enough.
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