I took this shot at 600mm (300mm + 2x TC III) as the Kestrel flew over my garden today. I've put in a 100% crop below the shot as I took it so as to show the IQ:
Distant Kestrel by 2slo7, on Flickr
IMAGE LINK: http://www.flickr.com/photos/2slo7/9387272880/
Distant Kestrel 100% crop by 2slo7, on Flickr
Edit: I took this to be a juv Kestrel when I first saw the images. Now on getting some advice it's more likely to be a Peregrine. So I'm delighted, a first for me.