The Twelve Days of Christmas Track

What is a 'colly-bird' in the context of the traditional English song 'The Twelve Days of Christmas'?
'On the fourth day of Christmas my true love sent to me four colly-birds, three French hens, two turtle doves and a partridge in a pear tree.'
The first edition of the Oxford English Dictionary published the section covering 'colly' in October 1891. It reveals that 'colly' is, or was, an old English word meaning black, from 'coal'. And, it says, a 'colly-bird' is the Blackbird.
Lyrics and chords available from Folk Den Project.