Every morning they left to see what was happening all over the world, and at sunset they flew back to Odin to rest on his shoulders, whispering in his ears all that they had seen. On this account they also symbolize secret knowledge.

A curiosity: there is a similarity in classical Greek culture. The goddess Athena, who had a blind eye like Odin, sent an owl to the world every morning, which came back in the evening to inform her; thus the owl too symbolizes knowledge.