Mediterranean and Black-headed Gulls

Aberdaron beach, fresh water running from the river onto the beach providing drinking water for all sorts of sea birds which gather in great numbers.
Along the river Grey and Pied Wagtails are often seen on the walls and rocks.  Returning to the beach mixed Gulls feed amongst the rocks and even to-day a Grey Heron joined the gulls.  Mediterranean Gulls mix with Black-headed Gulls for safety and they all feed together, when disturbed they are more obvious in flight where the pure white wings are clearly seen.
Getting to grips with a new book I bought (on good advice) The Collins Bird Guide.  Juvenile Med Gulls are very similar to the BHG, very little difference separates them.
I want to search the estuary in Criccieth, Pwllheli marina and my patch Porth Dinllaen and Morfa Nefyn over the comming months for any gulls with a difference.