"performs a particular literal and practical function, without need to include any hidden meanings.'"
Surely these were a mixture of both Steve, and it is the arrogance of the literate to presume they have "occasionally work their way down into oral tradition" wonder why that "down" still manages to raise my hackles.
The crowing of the cock in songs like "Willie O" (Grey Cock - sort of) still carried supernatural significance to non-literate Travellers who, at the same time close the doors and cover the windows if they heard magpies, or turn back if they encountered three jackdaws on their journey.
They would make a point of stamping on stag beetles because "that little feller told the Romans where Christ was hiding".
This wasn't just a rural phenomenon.   
When I was working on the ships on Liverpool docks men used to bring live fowl to be slaughtered for their meals.
I've known old tradesmen threaten to walk off the job if it was brought on board live.
Not being superstitious, I've always found the need to rationalise some of the songs I' sang (not difficult - most songs do this easily), but I believe this to be a modern phenomenon.
Folklore surrounding birds, animals and insects pre-date Crhristianity and it is illoical to claim that these beliefs didn't make their way into the songs.
I'm sure you are well aware of my scepticism of the practice of trying to date songs by their earliest printed source.
Jim Carroll