I need some advise/help. Most of my pictures are of birds (99.5%). I currently use a Canon 7D mk II with Canon a 100-400 mm mk II. The pictures are good but I want to up the quality. I am considering going to a Sony mirrorless. Which would give me the best quality pictures, Sony A9 vs Sony A7R mkIV, when mated with a Sony 400mm f2.8 with 2x TC. Please note that with birding there can be heavy cropping.