Its because during the Boer war, a murder of crows commanded the British defence at the fort of St. Georgetown. Descendants of the troops from that battle still salute descendants of the original crow commanders. Sorry - isn't it magpies?

If you see a single magpie you're supposed to salute it, or you get bad luck. I've been told, though, that if two people see the same bird at the same time that this negates any bad luck. It comes from the rhyme, as follows (there are two different versions that I know of); One for sorrow, Two for joy, Three for a girl, Four for a boy, Five for silver, Six for gold, Seven for a secret, never to be told. The other version goes One for sorrow, Two for mirth, Three for a wedding, Four for a birth, then on as the first version.

One for sorrow, two for joy.... you salute a single magpie to ward off the bad luck (sorrow)