I like the low angle and the layered BG. Might clean up the gray areas in the white up top. The image is somewhat lacking in fine detail; you should have been able to get the bird substantially in focus at f/5.6 with a profile pose like this. The whites are nicely controlled.
Agree with Doug. I was a little confused with the DOF too. But I withheld my comment because I am not familiar with DOFs at those long focal lengths. Could your focus point be in front of the bird and the back side of the DOF range caught the head? Have you sharpened just the head in PP? Since you had plenty of head-room in ISO and SS, IMO, you could have stopped down a bit. and still got the BG blur as its quite distant due to the nice low angle.
Loved the shooting angle and BG. Looks like 2nd winter RGB.
Thanks for the comments. I thought it was not quite as sharp as it should be. Unfortunately this is probably the best picture of a Ring-billed Gull that I have. Many of the other pictures I took at the same time are a lot like this one. I believe I need to try and calibrate my lens with the 1.4x teleconverter on it. It seems the focus is may be a little too far forward. Maybe I can try sharpening the head a little more.