It is said that the word "calling" is a derivation of the word collie or colly which comes from the old word for black and has it's origins in the coal industry (such as a colliery or a collier). Thus four calling birds may well refer to four blackbirds.
|Fluffy singing blackbird|
According to the RSPB website the blackbird is no longer in stock, so maybe I now have a collectable! Thankfully real blackbirds are not out of stock living almost everywhere in the UK and after a decline between the 70s and 90s has returned to the Green List after at 26% increase between 1995 and 2008.