Osprey Picture advice?

Well I just got a 400mm lens a few days ago in search of a bird lens. It's the old tamron 200-400 push pull type. Yes I am poor and can't afford one of the new super lenses. I got this image that I really like but the bird was really far away when I tried to crop it I felt like it became to grainy.. This is uncropped SOOC... One of those pull over to the side of the road shots.. No tripod etc.. What should my first thought be? Aperture priority wide open (5.6 in my case) Iso 200? Or shutter 1/4000? I had bracketing on and tried A and S, but of course auto gave me the best result... I just figured out how to set auto ISO in all modes.. It was a normal bright sunny day...Any thoughts? Thanks