On the Tower's premises there live eight ravens. Legend has it that if the ravens were to leave, the White Tower, the monarchy, and the British kingdom would fall. During the bombings of WWII, only one raven was residing in the castle!
The raven became a symbol of The Tower's bloody past because of their tendency to gather around gallows.
Today, the ravens have their wings clipped to keep them from abandoning the Tower (no word on how this one fits into the legend.) The ravens are all cared for by the Ravenmaster, one of the Yeomen Warders who serve as guides and guards for the Tower.