|Are these calling birds?|
|Definitely not these guys...|
|source: Anne Geddes|
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: Hubpages.com]