We get birds at our back deck feeders which are about 10 feet from the doorway. I have used a full stack of Kenko tubes (68mm) to get the 500mm f/4 to focus that close and it works well. Most times though, the 100-400 II works better as I can handhold it more easily to keep up with the birds. Attached is a photograph using the 500mm f/4 on the new 7D MK II and extension tubes. ISO 500, 1/1000 @ f/5.6, +1 EV.
Wow, splendid shot!