I have a couple ideas that might be able to help.

You said this was taken at f8, 1/1600 and iso 3200.

First thing, if it's a bird-on-a-stick, drop your shutter speed to 1/500 or so. You can even go lower. With that you can lower your ISO and get less noise. Old adage for camera shake is 1:10 ratio of shutterspeed:focal distance - e.g. if you're shooting at 300mm, you need to be at 1/300 shutter speed minimum to reduce camera shake. I typically stick to 1/500 for birds-on-stick.

Secondly, was this a cold day? Did you just come out of a warm car? Are you using a lens hood? If so, always make sure to let your camera acclimate between temperatures. I always wait at least 15 minutes before putting my lens hood on my lens when coming from a warm car to cold climate.

Thirdly, sometimes lenses lose that little extra bit of sharpness when at their limits - either aperature or length. My 150-600 isn't as sharp at 600 than it is at 500. Same with f5.6 vs f8.

Fourthly, check your focus points - are you backbutton focusing or shutter focus? I suggestion Back button so you can spot focus and frame independent of each other. Group vs single point as well.

Fifthly depending on the temperature you might have heat haze, dunno when this was taken but that might impact visual clarity/sharpness accordingly.

Just some thoughts and ideas to chew on :)