No such thing as specifically a seagull it is true, but a widely used and commonly accepted collective noun for a group of birds including herring gulls, common gulls, black headed gulls, etc. The common factor is 'sea', as they are all birds whose natural habitat is the seaside. We may be...
If they can sink their parrot beaks into seagulls' throats they'd be very welcome in Gloucester! (I like seagulls at the seaside but inland they're just sky rats).