I've never been able to get as close as I would like with harriers and kestrels. Despite the fact that they fly around looking down so much, I can never sneak up on them. As soon as I get to where they were, they fly away to where I was.
The first two images are like what I normally get. I scared off that harrier one too many times and it never came back. A second one arrived and nearly flew into me (images 3 & 4). Staying stationary works too if it doesn't know you're there already.
Again, sorry for the background - I will eventually get one close against the one or two clear backgrounds at my "hawk fields".
All shot with the 40D and the 300mm + 2x TC.