Hi peter - According to 'Brewers' the proper name is 'colly' and it's an old popular name for the Blackbird. "The Fourth day of Christmas my true love sent to me, Four colly birds, Three French hens, Two turtle doves, And a partridge in a pear tree".
Although in most versions the text reads 'Calling birds' I'm sure that the original line referred to 'Colley Birds' Sorry this doesn't answer your question Peteroops but maybe it will help with the research. I've done a quick search of the web but come up with nothing as yet.