Hard to say from these photos but I agree with
@Rassie that your lens is likely fine. A couple of thoughts:
- Birds in flight are hard subjects in the best of conditions so probably not the best starting point for evaluating lens sharpness and contrast. A touch too slow shutter speed or focus point landing on the wrong part of the bird as it flies can lead to thinking the lens is soft when it's more of a technique and tracking issue. I'd evaluate the lens quality on more stationary subjects with careful focus on the eye and if that all checks out then I wouldn't worry about the lens and instead focus on all the other things it takes to get razor sharp BIF images.
- Similarly these images are all backlit against a relatively bright sky which makes critical evaluation difficult. If you had to open up the shadows to bring up the exposure on the underside of the birds then that also tends to rob contrast and sharpness and can lead to thinking there are lens troubles when it has more to do with difficult lighting and what happens with large shadow pulls in post. Overhead birds are really tough this way as they're typically against bright blue skies or worse against bright overcast skies which is very challenging from an exposure standpoint. Again I'd do lens evaluations in better lighting situations so you can more easily see the lens qualities without dealing with things like strong backlighting. Also crops never improve image quality so that layers in more things that make it hard to evaluate the lens itself. Sometimes we have to crop, but in terms of evaluating a new lens I'd start with full frame filling images in more controlled settings.
