See Discussion here - http://surfbirds.com/forum/showthread.php?t=7147
Josh Jones has proven beyond doubt that both GREAT BLACK-BACKED and KELP (CAPE) GULLS are resident alongside each other at the Atlantic coast wetland of Kniffiss Lagoon, some 400 or so kilometres south of Agadir. This is a remarkable revelation, particularly when one considers that GBBG breeds no further south than a few rocky islets on the Brittany peninsular in NW France. Josh photographed two different adult KELP GULLS but it seems likely that the other eight adults usually seen by Western Palearctic visiting birders are actually GBBG.