There are also captive Ravens that nest and live in the Tower usually a minimum of six (I didn't see any in the photos but they are often there flying about, a bit of Odin going on there!). Its said that if they ever leave the Tower or fly away, the Tower and Monarchy will fall and be no more....
The collective noun is 'an unkindness of ravens'.
From historic-uk.com:
"The first Royal Observatory was housed in the north eastern turret of the White Tower. Legend has it that John Flamsteed (1646 – 1719), the ‘astronomical observator’ complained to King Charles II that the birds were interfering with his observations. The King therefore ordered their destruction only to be told that if the ravens left the Tower, the White Tower would fall and a great disaster befall the Kingdom. Sensibly the King changed his mind and decreed that at least six ravens should be kept at the Tower at all times to prevent disaster."