On Jun 22, 2020, Paul Carmichael wrote
(in article ):

On 22/06/2020 08:58, Paul Carmichael wrote:
On 21/06/2020 21:24, me wrote:
On Sun, 21 Jun 2020 15:23:09 -0400, wrote:

On Fri, 19 Jun 2020 13:13:19 +0200, Paul Carmichael
wrote:

The Sigma 120-400mm 5.6 is the only tele/autofocus that I have. My other lenses are
mostly
Samyang primes.

FYI, Review of the lens at imaging-resource.com
https://www.imaging-resource.com/len...sm-apo/review/

Sharpness
The 120-400mm is a fairly sharp lens, but its performance is optimized
for the wider range of its focal length spectrum. As you zoom in
towards the 400mm range, image sharpness degrades significantly.

and

Between 300 and 400mm, performance when used wide open (ƒ/5.6) is
nothing to write home about - very uneven and soft (3-5 blur units at
300mm, and 5-8 units at 400mm). Stopping down to ƒ/8 or ƒ/11 is
necessary to get any kind of sharpness out of the lens at this focal
length. Interestingly, image sharpness is better at ƒ/22-29 than when
used wide open at these focal lengths.

Wow. Time to play...

OK. I just went out onto the patio and took a few snaps at 400mm of stuff around the area.
Here they are straight from the camera. I haven't touched them, so they're big.

http://185.219.27.69:1961/tmp_delete_this_soon/

There you go!

Those are just fine, and there seems to be nothing wrong with your Sigma. However, it seems to me that the blue, clear sky background field removes much of the foliage distraction shown in your problem bird shots. Note, the best of your bird shots was the one with the clear sky background. That makes me think that for the bird shots the focus point had missed the target leading to those soft results.

At least you have some answers you can work with. Just remember that f/11 is not going to be the answer for all shots, and you will have to adjust all settings for different shooting environments.

--
Regards,
Savageduck