This is common with white birds...it's simply overexposed, and the overexposed part can't have any detail. When shooting white birds surrounded by darker backgrounds, you need to use -2/3 or more, EV; I'd guess a -1 for this photo would work.
Wonder if a UV or polarizing filter would help. Looks as though the camera focused on the foreground as well as got the exposure reading. I would have focused on the 2nd gull from the camera since thre will be more DOF behind it and the bird in front would be more focused. Less will appear in focus in front of the focus point. Have you tried bracketing.
Nice composition.
