Fab shots, you seem to be getting much better results with fast subjects than I am but with the same kit.
Probably down to your better skill, but can you share your AF settings and technique?
Thanks Lester, now you're asking...hmmmmm
When there is reasonable separation from the background I tend to use auto selection 65 point AF with an initial AF point turned on. For the swallows though, because they were so close to the background, I went with the basic expand area AF and instant lock on to a subject in frame.
Tracking via the viewfinder was almost impossible, many of my successful captures were taken by just pointing the lens towards the bird (I was generally lying down in the sand).
For some shots I pre-focused (the head on shot being an example) and waited for the swallow to come to me. The original 150-600mm's not the fastest so sometimes you just need a work around