First, use the longest lens possible. I just got the 500 f4 and have been learning to use it on songbirds. Since these guys are so small, I've been adding the 2X TC as well. For ducks and other larger birds, the 500 does fine and sometimes I add the 1.4 TC.
I moved my feeders in the yard so I can have a good view from a small window. Then I set up my tripod and camera at the window, slide it open and have a ready made blind, at least sort of. Since thel window is small, it works well and lets me get close. This Pine Siskin was about 20 feet away and the photo is cropped some to eliminate the side of the feeder.
This shot of the Pine Siskin in a tree about 35 feet away, is not cropped, and is at 1000mm (the 500 and the 2X TC).