The mediterranean gull is one of those species, like the little egret, that was a real rarity here forty or so years ago but are now well established here and breeding in favoured locations. I remember going to Titchfield Haven in Hampshire one Saturday in the early 1970s and was surprised to find thirty or so 'birders' there. On enquiry I discovered they had come to see one of the first mediterranean gulls to be recorded in Hampshire; a photograph of it subsequently appeared on a book produced by the Hampshire Ornithological Society with details of all the species recorded in Hampshire. I think the book was called "The Revised LIst of Hampshire Birds"; sadly my copy has long since gone from my library as I must have loaned it to someone and never got it back.
The 'med' gull is now resident in Dorset and breeds on the Brownsea lagoon and possibly at other sites too. As the weekly chart shows rarely a week goes by without at least one report however it can be seen from the chart that there are peaks during the spring and autumn migration periods which shows our resident numbers are substantially inflated by migrant birds. This seems especially true in 'autumn' from week 25 at the beginning of July through until week 31 at the end of August. In these weeks it seems a large number of juvenile birds are recorded amongst the adults so we are probably seeing the dispersal from nesting sites of fledged families who are out learning the ropes with their parents.
The distribution map shows just how widespread the mediterranean gull now is in Dorset with records from all of the regular coastal spots as well as from some inland sites and even as far inland as Cranborne.
The nesting colony on Brownsea Island is by far the best place to see them; take the boat across in June or July and see them on their nests and feeding their young just a matter of feet away from you.