Banded Grey-headed Gull at Modimola Dam

While birding at Modimola Dam this afternoon I came across this Grey-headed Gull in its breeding plumage with a metal ring.I am not aware of any ringing activity within Mafikeng and my guess is that it must be from somewhere within the country or from across neighbouring Botswana. I wish I could know what distance Grey-headed Gulls could travel. Anybody with an answer? There is quit plenty of the at the dam and this is my first sighting of one with a ring.

A breeding Grey-headed Gull with a metal ring

As the sun was setting it was still perched at the same spot