So majestic in flight. When I am photographing sea gulls or any bird in flight I always make sure my ISO is between 200 and 400 depending on the conditions (such as overcast), on dark cloudy days I may need to bump up the ISO a little higher in order to achieve the high shutter speed, ranging somewhere between 1000 and 1200. Better to have a little noise than blurry images. Always keep in mind the exposure has to be perfect in order to keep from blowing out the highlights in the sky and the bird itself. Experiment with you exposure to assure a good outcome. Have fun and chase the gulls, they are definitely entertaining.