Barn owl's are extremely difficult bird to photograph, I'm finding. They are small, fast, low flying, and white. Even with a teleconverter fitted, and using a crop sensor camera body, it's hard to get close enough to get a decent shot with my 500mm lens. Tracking them in flight is a real challenge too - the auto focus on my D300 has a hard time keeping up. Exposing correctly, to avoid blowing the highlights, isn't easy either - especially in direct sunlight.
After plenty of attempts, my favorite shot is the one shown on the front page of this blog. Here's another, taken on the same evening back in February.
Nikon D300 + 500mm f4 + 1.4x TC @ f/8, 1/1000th sec, ISO 400
Next weekend we are heading up again. For the last couple of visits the weather has not been good, so hopefully there will be some decent light this time.