The Birds of Rhiannon are mentioned in two of the texts included in the collection of medieval Welsh tales known as The Mabinogion. In the earliest of these tales – Culhwch and Olwen – the giant Ysbaddaden Pencawr says he wants the Birds of Rhiannon – “they that wake the dead and lull the living to sleep” – to entertain him on the night Culhwch weds his daughter. This is one of the many seemingly impossible tasks he sets for Culhwch as a condition of the marriage. Although the tale does describe how several of the tasks are achieved, it does not describe all of them and there is no mention of how – or if – the Birds of Rhiannon are obtained. Of all the tasks set this would require more than the skills and ingenuity of Arthur’s men that enables most of them to be completed. We learn no more about the birds in this tale.
But they are also mentioned in the second of the sequence of four Mabinogi tales. On their return from Ireland with the head of Bendigeidfran the seven survivors of an expedition to rescue Branwen hear the song of these birds far out over the sea. Bendigeidfan had told them that this would be so before they returned: “You will be feasting seven years, with the Birds of Rhiannon singing to you”. So it was, and when they heard these birds there were three of them “and whatever kind of song they had heard before was unpleasant compared to that song”. The birds sang from far away “but their song was as clear to them as if they were there with them”. Here the song lulls the living feasters into a state of timeless suspension for the seven years of the feast and then for a further eighty years with the head of Bendigeidfran on the island of Gwales. When they return to the world it is as if no time had passed and events continue as if they had just left it, and all of the pain and trouble of the world returns to them.
So the song of these birds is a song of enchantment, bringing those who hear it into a state of bliss. They are otherworld birds. For Culhwch to have brought them to Ysbaddaden would have been a feat to outdo all the other marvellous tasks that were fulfilled. Yet, we are to suppose, the birds did entertain the wedding feast and lulled Ysbaddaden to sleep when, like Bendigeidfran, his head was struck off and he left the world for an otherworld repose whose time is not yet ended, if it has even yet begun.