It's 'colly birds', hurr-durr.
I would think that, as much of a role as they play in the story, that SOMEONE would have advised the writer and actors that the item in the song is 'colly birds', not 'calling birds', and that saying/singing 'four calling birds' is not only completely nonsensical but generally considered to be lazy and ignorant...right up there with asking José if he can see in the beginning of the Star-Spangled Banner. Awesome fumble, Hallmark.
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