After the Great Fire in 1666, scavenging ravens became a huge problem in London, and an initial attempt was made to eradicate them.  However, Royal Astronomer John Flamsteed explained to King Charles II that killing all ravens would be a bad omen, and that the kingdom would not outlive the last killed raven. Charles II then ordered six birds to be kept at the Tower.  There have been ravens in the tower ever since.