Chlidonias leucopterus
This palearctic summer migrant is the smallest of the lake terns and can be distinguished from black and whiskered terns by its contrasting white rump (in all plumages).
I had seen this species at Stanger (North Coast KZN), Marievale Bird Sanctuary (South East Gauteng) and Mkhombo Dam Nature Reserve (Mpumulanga), but had to wait until March 2013 before I could get any reasonable photographs. These were taken at a dam on the R550 near Devon.