While out shooting this morning, I decided to do a bit of a comparison. This is the same Kestrel on the same sign, shot with the same camera at the same settings. Those settings were 1/1600 Sec, f/7.1, ISO 500, 400mm. So, what's the difference? Each image was shot with a different lens or lens and teleconverter.
This first image was shot with the Canon EF 70-200mm f/2.8L IS II and the EF 2.0x III Extender
This image was shot with the Canon EF 100-400mm f/4.5-5.6 L IS II
Last but not least, this image was shot with the Canon EF 400mm f/2.8L IS II
Click on the images to make them bigger and poke around. I'll let you decide which one you prefer.