To this day, one of the best-known couplets in popular music is that immortalised by Forces sweetheart Vera Lynn in her 1941 recording: "There'll be bluebirds over the white cliffs of Dover".
The lyrics were written not in Britain but more than 5,000 miles away in a studio in Los Angeles by Nat Burton.
But to what species of 'bluebird' was Burton referring?
This question is explored by author Nicholas Milton in his informative and entertaining book which starts with a chapter on the morale-raising use of birds in the popular culture of the time.
After ruling out both blue tits and swallows, two commonly-seen native species, he thinks Burton (who was not known for his knowledge of birds) simply chose one of the American bluebird species because they are regarded in the US as "synonomous with joy and love".
But, intriguingly, he also speculates that they "aren't feathered birds at all but war birds" - specifically Spitfire fighter aeroplanes whose undersides were painted blue to provide camouflage when seen against the sky.
This is a thoughtful, excellently-researched book full also of unique perspectives about wartime ornithology and its pioneers (including Richard Meinertzhagen, James Fisher, Peter Scott, E.H. Ware, Eric Hosking, Viscount Alanbrooke, Richard Fitter and many more).
The book is published at £25 by Pen & Sword History (www.pen-and-sword.co.uk)