Thanks for the advice. They look fairly sharp on my screen but they have been heavily cropped. I think I will set the shutter speed and aperture manually the next time and leave the camera on auto iso. The d7100 is ok until 1600 iso.Nice start. At least on my screen, these do not look absolutely sharp. I shoot with the tamron 150-600 g2 a lot.
You need a lot more shutter speed than 1/500, especially with the crop sensor. There are several ways to get more light.
On that lens, with bif, you do not need f8. You can shoot it wide open. That gives you a stop of light.
You do not need to stay at iso100. Do your own testing, I have never shot with the 7100, but I bet you can go to iso 800 or more with great results.
You need to get the shutter speed up, 1/1250 or more. I actually like 1/1250 when I am handheld shooting a still bird on a stick, 1/3200 gives me great birds in flight when I can get it.
Birds in flight are not easy. Lots of excellent photographers on this site throw up images and make it look simple, its not.
You are off to a good start, but I think you will be even more successful if you look at some of the suggestions above.
It is not the lens, you can look at some of my birds here, which are almost all shot with the 150-600.