I'm a huge fan of nature shots, and having shot these swallows myself I know how tremendously difficult it is to get one in flight and even nearly sharp! Therefore I can appreciate this shot before even thinking about critique.

My first thought is 'flash damn it' (if you have one of course) ... I always use an external flash if I'm serious about getting birds. The flash is fast, faster than those birds, so if you marry it to the right shutter speed the flash will help freeze the action. In a scene like this it would also have lit the bird up more and helped isolate it from the background which is quite distracting. Unfortunately swallows tend to hang around man made objects so clutter in the background is common. Again, the flash will help you to use a slightly lower shutter speed to help with background blur, but give you a fighting chance of panning with, and getting the bird sharp. Obviously the flash will also highlight the birds features better. The downside is you will lose much of the side light you caught here.

After all that, you could duplicate this frame and add a gaussian or slight movement blur to the upper frame, adjust levels/curves/dodge the bird a bit on the lower frame, then create a layer mask and blend the brightened bird through. That would help isolate the bird in a post process.