"There'll be bluebirds over the white cliffs of Dover" scans. I don't know much about music, but I can hear a lyric that fits a melody. I one remember reading a piece written by Bill Oddie, giving a masterful explanation as to why the song came to be called "A Nightingale sang in Berkeley Square". As he pointed out, shearwater would have been surreal and flycatcher distinctly suspect.
Blackbirds would scan and are native, but black isn't a hopeful colour, and there was a need for hope in 1942.
And of course it should be noted that the lyrics to “White Cliffs of Dover” were written by an American who would have been much more familiar with bluebirds than an English songwriter would have been.
There was an earlier successful American song “Bluebird of Happiness” which probably inspired the lyricist to feature bluebirds in this song.
