On a previous post I commented about totemism and shaman practices by ancient Celts. Celts held no frontiers between human and animal forms and they were in no way in lower status than humankind. Birds were specially worshiped as messengers of the Otherworld, being capable to fly breaking their bondage to earth, and certainly the Raven and the Swan are amongst the most precious ones.
In Wales bands of warriors at the battle of Cattraeth are described in Aneurin's Gododin poem as dogs, wolves, bears, and ravens, and the quotations on the Mabinogion amongst other instances, the wonderful crows of Owain, prince of Rheged, a contemporary of Arthur, which always secured factory by the aid of the three hundred crows under its command, which in fact may have been a clan with the raven as their totem, sometimes misunderstood as actual ravens.
Not only the goddess Morrigu is associated to this particular bird, but also the god Bran the Blessed (Welsh: Bendigeidfran or Brân Fendigaidd, literally "Blessed Raven").
Power is one of Bran’s qualities and famous for his succeeds on battle, maybe that’s why his name means 'crow' or 'raven' , associated thus with corvids because of these bird dietary habits which include dead corpses.
It is recounted that his head is said to be buried under the Tower of London facing toward France, a possible origin for the keeping of ravens in the Tower, which are said to protect the fortunes of Britain against invaders.
The raven was the ruler of the domain of air and therefore of communication; the cry of the raven was often interpreted as the voice of the gods. Images of the gods Lugh and Bran often depict them with birds alighting on their heads and shoulders, symbolizing this divine communication. (The Norse god Odin, who is sometimes compared to Lugh and Bran, has as his companions two ravens called Thought and Memory.) For this reason, ravens were favored by the druids for use in divinatory ritual.
In Cornish folklore crows and particularly magpies are again associated with death and the 'Otherworld', and must always be greeted with respect.
J. A. MacCulloch supposes that it was because of their dark color and gruesome dietary habits, that ravens were especially connected with gods of war and death.
It was the raven the one who accompanied the souls of the dead to the afterlife, and portraits of the deceased often depicted them with the bird. Ravens were sometimes viewed as reincarnated warriors or heroes , like the cited Mabinogion hero Owain, who had an army of invincible ravens, which are sometimes interpreted as an army of reincarnated warriors.
The origin of 'counting crows' as augury is British; however the British versions rather count magpies - their black and white pied colouring reflecting the realms of both the living and the dead.