Picture sharpness on Canon M6 ii with Tamron 100-400mm

Polisky wrote:

I hoped this lens would allow me to exploit the M6's high resolution to the fullest and really crop a lot (those kingfishers tend to be on the other side of the river...)

Same here! I was using the 55-200mm for quite a while and I thought a higher quality lens with double the focal length would mean I could be double the distance. Technically it does, but at the end of the day getting the bird to fill the frame is still a challenge and the extra focal length only makes it easier, doesn't remove the problem entirely.

As for the experiment you're proposing, is the idea to have a relatively small subject so that it is clear where the focus is supposed to be, and then check if the focus under/over-shoots as I move the subject further away for each shot?

I'd be very curious to try it, just wanted to understand what the aim would be!

Was not thinking anything as scientific as that - although that would be a great way of doing it! The thought was more to do practice shots with a subject that you can get closer to and in the style you want and see whether the lens is holding up to your expectations. Could be a toy, could be some easier subjects (sparrows, etc). If you are getting shots where the subject is a good proportion of the frame but they still aren't sharp then I think that points to some possible issues.

I did some field work myself where I learned I am horrible at remembering distances, I passed by where I took that first shot of the Honeyeater today and measured that I was actually 4 metres away, and the bird is about 2cms larger than a sparrow for comparison. So don't feel that you need to be getting super close especially for a larger bird, you can still get great shots from a reasonable distance.

Duade Paton has a great Youtube video going over some of the fundamental tips that drastically improve the quality of your shots, his advice has been immensely helpful for me so hopefully you might find it helpful as well.