At the tower there were serveral ravens that lived on the grounds. Over time, the kings and queens began to believe that their authority to rule came from the birds and as long as there were 6 ravens at the tower, they would have power. Although claiming not to be superstitious in the current day, the Beefeaters have 8 ravens that live at the tower who are fed daily (and have their wings clipped so they can't fly away). Nice.