Have you ever wondered what those "Four Calling Birds" are all about? (Explanation to follow...)  

Are these calling birds?

Definitely not these guys...

Hmmm. [source]

My favorite:
source: Anne Geddes
There's a reason the stanza about "calling birds" doesn't make much sense: the original line in the song The 12 Days of Christmas names four "colly birds", an alternate word for the Common Blackbird. The Blackbird is a common backyard bird in Europe and has a melodious song. It also lives in Asia, North Africa and it has been introduced to Australia and New Zealand. [source:]

In England a coal mine is called a colliery and colly or collie is a derivation of this and means black like coal. 

For a long time in England, blackbirds have been referred to as both blackbirds (as in the nursery rhyme Sing a Song of Sixpence) and colly birds as in The Twelve Days of Christmas.

As to why the person in the song would give his true love a gift of blackbirds, the answer is that this would have been another gift of food. Blackbirds were plentiful and were a common food. [source:]

Mystery solved.