Great capture! I don't see EXIF data in the image (post-processing often strips it out), so I don't know the lens or ISO. It seems just a trifle noisy. Maybe you had to crop it a lot. You could try noise reduction software to clean it up some. A heck of a shot!
I didn´t crop it. It was early morning and i had to put ISO at high levels to maintain shutter speed at 1/500. By the way, the lens is a Sigma 150 - 500 and the DLSR is a Nikon D90.
We've had various discussions before around here about high ISO and noise: better to get an interesting shot like this and accept a little noise than to say, "Gee, it will look noisy, I guess I won't press the shutter button." Also that it's better to raise the ISO to get a proper exposure rather than underexpose, because it will be even worse if you push it up later. No argument with what you did. It still might be worth trying some noise reduction in post-processing (PP), if you haven't done so already. The built-in tool(s) of whatever you use for PP, or an add-on like NeatImage (which costs something like $30 US for the base version). I know there are lots of others, but NeatImage is the only one I've used.
I have no doubt and i rather take te picture with "noise" then not to take it! As is a normal lens (aperture 6.3 at 500 mm) shutter speed must be al least equal to FL so 1/500s is the minimum to achieve normal results, despite having sigma´s VR.