so, why would you give your true love 4 crows? it would have been a gift of food. blackbirds were plentiful and a common food at the time. we see them again in the nursery rhyme - sing a song of sixpence, we see them being served in a meat pie. this may have been the way they were commonly served.
in the christian tradition the 4 colly birds symbolize the four gospels or evangelists - matthew, mark, luke and john.
so -- we've (myself included) have learned something new today -- and been singing the carol wrong all these years. tune in tomorrow and see what other surprise i might dig up! :)