birds perched on the branches of the World Tree represent men's souls. Because shamans can change themselves into 'birds,' that is, because they enjoy the 'spirit' condition, they are able to fly to the World Tree to bring back 'soul-birds.'
Eliade establishes the bird as the primary shamanic symbol of the soul in its liberated, spirit-state, linking ornithomorphic transformation to the World Tree cosmology and ancestor-soul mythology across Eurasia.
, Shamanism: Archaic Techniques of Ecstasy, 1951thesis