ajohn

It's definitely an immature bird - the underparts and underwings are quite dark and with darker mottling. An adult of any species should be largely white below.
So it's definitely not the (adult) bird you linked to from last spring.

I'm not entirely sure about ID. Like John H I don't think it's Black-tailed, and Heuglin's is probably the most likely. Best to leave it as gull sp. I think.