And the answer is...

As Kate correctly answered:

Gwylum, Thor, Hugine, Munin, Branwen, Bran, Gundulf, and Baldrick are the ravens in residence at the Tower of London! I love legends and myths about England, and this is one of my favorites. (Another favorite is the ghost of Anne Boleyn strolling about the Tower Green with her head tucked under her arm...)

From Wikipedia.com:

There have been at least six (there are currently eight) ravens in residence at the Tower for centuries, although the exact point in history when they arrived is unknown. Charles II ordered their removal when he discovered their droppings all over his telescope. However, they were not removed because Charles was then told of the legend that if the ravens ever leave the Tower of London, the White Tower, the monarchy, and the entire Kingdom would fall. And Charles, during the time of the English Civil War, superstition or not, was not prepared to take the chance.

The only time ravens have been recorded absent at the Tower was in 1946, shortly after World War II, when the Kingdom was close to falling.

The ravens' wings are now clipped to prevent them from flying away.

(God save the Queen! And Mother England!)