BTW...I am seeing a few people lose their minds over f/9. Granted, I would have preferred better, but can we stop for a second:

400 mm f/5.6 the DoF (according to Photopills) at 30 ft is 6.7 inches
500 mm f/4 the DoF at 30 ft is 3"
500 mm f/7.1: 5.3"
500 mm f/4: 2.1"
800 mm f/9 the DoF at 30 ft is 2.5"

So, only the 600 mm f/4 has a shallower DoF? In the images I am seeing, if the background is sufficiently far behind the subject, this lens is providing some nice bokeh.

BTW, my bird feeder is ~30 ft from where I shoot.

I understand light, but f/9 is only 2/3 stop slower than f/7.1....I'll take 300 mm extra focal length range for 2/3 stops of light....and I'll take IQ over either.