Ravens of death

The ravens of death were mechanical ravens, amongst the arsenal controlled by Kate Stewart of UNIT. (TV: The Power of Three) They were apparently stationed around the Tower of London. Malcolm Taylor was in charge of replacing their batteries. (TV: The Day of the Doctor) They are also able to reproduce. (Webcast: The Zygon Isolation)

Behind the scenes
The ravens of death may be a reference to an old superstition behind the practice of inside the Tower of London. The superstition goes that "if the Tower of London ravens are lost or fly away, the Crown will fall and Britain with it."