There is a lot of Irish/Celtic folk lore about 3 impossible things, birds in general and crows in particular. "Heroes" are asked to do 3 impossible things to win the prize, (and the number 3 has magical meanings in itself). Many birds were sacred to specific gods/goddesses, and many folk heroes had personal prohibitions regarding eating, killing, etc. of birds. The crow was regarded as a Druidic/bards bird, as it had the gift of prophecy. Maybe some of this has a bearing on the song?