This is cool, and I know that it is very hard to get birds while they are in flight. You definity have to pan them. I think the image would have worked better if you could have added the panning in with a slower shutter speed.
The panning effect is too subtle for me. Try a longer shutter to accentuate the feeling of motion. It would also help to have something in the background that shows motion better than clouds.
Because of the uniform background there is no indication of motion from the background. The birds show a little bit of motion but that could also be from just being out of focus.
to me, this challenge was about conveying the sense of motion. you've succeeded in capturing the birds, but I don't feel any motion at all. capturing a background where the motion was evident would've helped.