Summary: The Tower of London keeps a group of ravens permanently on the grounds because of a legend: should all the ravens ever leave the tower then it, the British crown and British kingdom will fall.
The Legend: We don't know how the ravens' legend really began, but the official story is as follows. When John Flamsteed, the astronomical observer of Charles II, tried to take measurements from the White Tower he encountered interference from ravens who lived amongst the Tower's towers. Exasperated, he appealed to the king who ordered the ravens killed. However, someone unknown warned Charles that should all the ravens ever leave, the White Tower would fall, and with it the British monarchy and kingdom.
Charles, or whoever really did first hear about the legend, evidently believed it, for they ordered that six ravens were to be kept at the Tower at all times (the rest could be moved on). Since then the legend has expanded to tie first the fate of the British Empire and later the Commonwealth to the ravens. Given the high stakes, the ravens are now encouraged to stay by clipping their wings so they can't fly off, a process we are assured is painless.
The Ravens: Six ravens, plus a number of reserves, are still kept at the Tower; the current group comprise three males, called Gwyllum, Bran and Cedric, and three females, called Hugine, Munin and Branwen. The latter was named via a competition on a leading national children's television program. Cared for by a dedicated yeoman warder called the Ravenmaster – with whom they usually bond - they receive a healthy diet of raw meat, special biscuits, eggs and the occasional rabbit.
In past centuries new recruits were drawn from the wild, but recent replacement ravens have been drawn from animal sanctuaries and the Tower has its own breeding program. Given that the ravens are now a national icon for the British, and that many tourists make a special effort to see them, some ravens have been replaced because of unruly behaviour. There are also still a few escapes; 'Grog' was famously last seen outside a London pub...
How close has Britain come?: Officially at least, the ravens have been a continuous presence at the tower since Charles II's orders. The closest they've ever came to being absent was during the Second World War, when the Tower suffered heavy bombing and only a single raven remained (called Grip). However, the sheer level of symbolism within this does make me wonder if it's true!
