To offer a little clarification on the matter of the tune most usually sung to 'The Twa Corbies'-
I first learned this ballad in 1957/8 at the Ballads Club in Rutherglen Academy, near Glasgow, which was started by our English teacher Norman Buchan, and we sang it to the Breton tune which had been matched to it by another English teacher Morris Blythman who taught in Glasgow (and who had been Norman's best man...). Both men were certainly familiar with it as a poem, but when Morris heard the Breton tune on holiday, he made an immediate connection.
This is what Norman had to say about it in the notes to the Wee Red Book -- as "101 Scottish Songs" was known amongst the folk community when it was published by Collins in 1962 --
'This is one of the greatest of all our ballads. But a ballad is only a ballad when it is sung. It lacked a tune, and I had never heard any successful attempt at providing a setting for it. This tune, an ancient Breton war song, was taught to Morris Blythman by the Breton folk-singer Zaig Montjarret, and he set the Scottish ballad to it. The result is astonishingly right. There is a curious submerged lilt in the tune which exactly sets the mood of the poem, with the jaunty chatter of the crows as against the macabre theme of their talk.'
Norman had used this musical version of the ballad in our second year English class, when I was aged 13, and set us homework to rewrite a modern version with the rhythms of the tune in our head. I wrote a ballad of a meeting between a young man who was pursuing a lassie who was promised elsewhere, but when she tried to refuse him gently, he stabbed first her and then himself, gathering her corpse into his arms for the finale -- "He pit his airms roon that lassie fair oh / An' she was his for evermair oh x2". (Remember, I was only 13, so what did I know of passion or anything else!)
Anyway, Norman published a Scottish traditional song in the Weekly Scotsman newspaper, every week for a year as I recollect -- around 1959/60 -- and included both 'Twa Corbies' and my version (together with other great songs such as 'The Gallowa' Hills', 'The Wark o' the Weavers', 'Coulters Candy', 'Birnie Bouzle', 'Tullochgorum', 'Come a' ye Tramps an' Hawker Lads', 'MacPherson's Rant' etc.). This then became the basis of the later publication of "101".