jmc wrote:

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?

You don't want to compensate; you want to meter. Put your camera in
manual mode, find something to meter off, and then leave the exposure
settings alone. This way, you won't have to change any settings whether
you're shooting against the woods or against the gray-blue sky.

You could also try spot metering, if you're sure the bird is in the
center of the frame, but this will limit what you could shoot.

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

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

Lovely.