Yes, you read that right. Colly birds, not calling birds.
Welcome once again to our 12 days of Christmas. So what is this about colly birds then?
Well, the word 'colly' is an old word, related to one still in use, colliery. That is, a coal mine. Colly means as black as coal. So a colly bird is in fact a blackbird. Which also sings nicely, and hence would justify as a calling bird. So I guess it works out ok after all…