Thanks all for your comments. With the Grebes there is a bit of ritual behaviour leading up to this shot so you do get some warning of when it's about to happen. Isac, yes the 800mm does come in handy because the birds never read the script and come in for their close up, I see you've got a Sigma 500mm, with the crop factor of the 7Dii it should give you the same reach as the 800mm on a full frame camera.
