Four Calling Birds by Richard Rodney Bennett

"This is an attempt to decipher the mystery that is the song 'The 12 Days of Christmas'. You've come to the page that is concentrating on the line Four Calling Birds, and here's what our Researchers came up with when we asked them what on earth this line meant.
For those of you with an interest in etymology, the term 'calling birds' in the song is a deviation from the original term 'colly' or 'collie bird'. 'Colly' means 'black' and came from the old word for coal, so the four colly birds in the carol are in fact blackbirds. This doesn't really explain why anyone should want to give their true love four blackbirds, but there's no accounting for taste..."
http://www.bbc.co.uk/dna/h2g2/alabaster/A212266