I have been reading about Nikon's f/3.5 400mm prime lens from 1976 (still often going for $1,000 used). It's a manual focus lens and I've started to realize that auto-focus on my Tamron 150-600mm G2 (or 850mm with my Kenko 1.4x teleconverter) is only so useful. For me, my Nikon's AF has issues with the teleconverter (an f/8 aperture is somewhat low-light/low-contrast for the D810's AF) and often things like trees or other foreground obstructions or tracking fast-moving objects can be difficult.
I had lots of time to sit and watch birds and had a blast trying to manually focus catching them. As seen by the results, in flight was a challenge but even catching them stationary seems an accomplishment at such close distance and as flighty as they were. Here they are among other subjects near and far. Most shots are at full focal length provided by the lens.
Well Grounded Material
Airplanes and the Airport are Everpresent Here
Birds of the Sky
Birds of the Natural Kind
Unusual Birds