Photographing birds in flight

I've finally purchased a longer lens for my XTi - the Canon 70-300mm DO
IS ISM. Nice lens, the shorter physical length is a lot more stable in
my small hands. I have been getting some very good pictures with this
lens, so I'm quite happy with it, despite the high cost.

I've been practicing taking pictures of birds in flight - raptors, for
the most part.

Not surprisingly, all of the ones taken with the sky as background, came
out with very dark birds. Also, for non-soaring birds, I'm finding it
very hard to follow them, and get decent pictures.

Is there a tutorial on the 'net somewheres that'll help me learn how to
take better bird action photos? How much do I compensate when the bird
is silhouetted against the sky?

Pictures where the bird's not silhouetted against the sky come out better:

http://www.photosig.com/go/photos/view?id=1959314

Thanks for any advice or information!

jmc