Hey, Jack that is one hell of a shot. What aperture was that - the AF looks about on the beak and both wings are definitely OOF so I am not sure if AF is the issue.
The problem seems to be a combination of movement in the bird and you trying to get the shot. It looks like a 'grab shot' to me (see the bird in flight, camera up, position and shoot - and if the bird is flying left to right, and your brought your camera to bear from right to left that will exacerbate it). And if this shot is full frame it seems the bird was quite close which will add even more complexity. All ways up, I think you have done excellently to get it.
I am presuming you were you not tracking the bird for a couple of seconds first?