Re: Birds in flight - disappointing. Technique?
I’m commenting from a cell phone, so these remarks are more general in nature for addressing soft BIF images and not specific to anything I see in your images.
Regarding VC, if you haven’t already done so, check the manufacturer’s recommendations concerning use of VC when shooting at high shutter speeds. With some lens/camera combos, you’re better off turning stabilization off when shooting BIF at high shutter speeds.
Have you checked the AF calibration for this lens with your D500? At long distances, it might not make much difference as increased depth of field at long range might mitigate it, but with my one and only D500, I had to calibrate every telephoto and tele/TC combo to get sharp images through correcting for excessive front or back focusing. I believe this was more of an issue with my D500 copy rather than D500s, in general. But it’s worth checking. I also needed to make adjustments with some of my lenses with my D850s as well. It’s relatively easy to do using the D500’s AF fine tune feature, though it will only work for one focal length. A Tamron lens dock should offer more fine-tuning options.
Finally, and perhaps most importantly, be aware of and try to avoid atmospheric conditions that lead to soft images. While everything can look perfectly clear to the naked eye, varying pockets air density and/or particulates between you and your subject can scatter and bend light waves to the point you end with mush. This frequently occurs when shooting in full sun from mid morning to late afternoon. As recommended by Bill Ferris, best to shoot BIF early and late in the day both to get better sun angles, but also to avoid unfavorable atmospheric conditions.
Thanks Alan. Some of very helpful ideas there. I have now done the calibration on the combination but haven't been able to try in the wild yet (the change was -4 so perhaps won't make much difference).