The Yellow-billed Duck is native to South and East Africa.
The subspecies shown above is the South African Yellow-billed Duck
(Anas undulata undulata).
The Yellow-billed Duck's "speculum" (wing window) is said to be green, as it appears from this angle . . .
. . . but from this angle the speculum appears to show blue (same duck, same time, different angle).
"What's up?"