I evidently failed to keep up with this thread after my second posting on 26 October 2012, when I said "Are there any birds other than the cock that are offered gold and/or silver for parts of themselves? That does strike me as a bizarre notion, but it must have come from somewhere."

Steve commented "What strikes us today as bizarre might not have done in the 18th century and earlier. After all in poetry, and this is a type of poetry, anything can happen. Talking animals are fairly common. Some of the ballad makers were quite simple people who didn't necessarily put in a great deal of thought or sense, and often were just imitating a simple idea from elsewhere, classical mythology, folk-tale etc."

Yes, talking animals are common in both highbrow literature and folklore (and remain so to this day in stories for children), but fitting a cockerel with gold and silver body parts still seems to me much more unusual and remarkable. I agree that imitation of the idea of the Roman votive practice seems more likely than the idea remaining current over all those centuries, but a connection of some sort is more plausible than a random invention of those gold and silver parts by whoever first introduced them into a version of the song.