White-headed gull flying with egg

I didn't realize until I viewed my shots on the computer that this gull is flying with an egg in its bill. Is this common behavior?

And the bill oddly looks hooked - but must just be some foreign matter on the egg.

Maybe it's some other bird's egg.

I'd guess it is some other birds egg. Pictures of a Gulls nest I saw once showed larger eggs than this with a mottled pattern. Gulls are scavengers, they will eat almost anything, including eggs.This one may well have raided some other birds nest to feed it's own chicks.
A good image in any event.