In Irish Celtic folklore, ravens were mediators between the gods and the mortals. They would bring prophecies and messages between the realm of the gods and Earth. The Celtic goddess Morrigan takes the form of a raven to visit those who will soon die on the battlefield.
The best raven myth is that of the Welsh/Celtic god, Brân the Blessed. The Welsh word for raven is bran. Brân the Blessed was the King of the Britons and was dying after a vicious battle against the Irish. He was so anxious to protect his people, that he told his followers to cut off his head and bury it under the White Hill, so that his presence would prevent Britain from ever being invaded. This is why the Tower of London (built over White Hill) is populated by ravens and has 6 official raven residents. Legend has it that if the ravens ever leave the Tower, the British empire will fall.
During World War II, only one raven was able to survive the hardships of the bombing during the Blitz, so the Prime Minister, Sir Winston Churchill, ordered more ravens to be brought in, in order to bring the flock up to the correct size. The Tower ravens are enlisted as soldiers of the Kingdom, and were issued attestation cards in the same way as soldiers and police. As is the case with soldiers, the ravens can be dismissed for unsatisfactory conduct.