When the song was written , by an American I think, the author did not realise that there were no blue birds in England.The are an American species.
It was decided to publish the song anyway rather than try to change it.
Vera Lynn's version of that song is, of course, the most memorable involving, as it did, an emotional tie between the British public and WWII. However, if you Google it you will find that many bands and singers performed it prior to her making it "famous".