This was when I had 4 EOS bodies (1DMk2, 20D, 50D, & 5D) and was making the decision on which one to keep as a primary/backup for my wildlife kit. I decided to rank them in a controlled shootout using a stuffed Ptarmigan.

I place the Ptarmigan in what I would call average stalking distance from the tripod - about 30 ft in late afternoon light. Although this already places an advantage to the higher pixel density 1.6 crop sensors over the (in this case identical pixel size and density) 1.3 or full frame - too bad. That’s life when shooting birds - you can only go so close and then they fly away.

With each camera I shot mirror lockup, fixed aperture with a 500 f/4. Took the best of each grouping and processed the RAW file in DPP to what looked best to me for color, white balance, sharpness, etc. I did further minor levels adjustment in Photoshop. The examples posted are at ISO400 - with common crops. The head shots are at 100% and the shoulder shots are crops that have been resized to 1500 pixels wide. Also included are resized pics with no cropping to get a reference to the magnitude of cropping required.