This is the world's most numerous wild bird, the Red-billed quelea. Researchers estimate the number to one and a half billion individuals. In this case, a flock came to the waterhole in the Etosha National Park, late in the afternoon, and perched in a tree - which suddenly seemed to change shape. Every millimeter seemed to be occupied by birds. The movements to and from the tree were massive and the only comparison I could think of, when the flock shaded the sun, was "the locuts of Egypt"...