Swans hatched during Thunder

The fable of the singing of swans at death is well known; but I recently heard a bit of "folk lore" as to the birth of swans quite as poetical, and probably equally true. It is this: that swans are always hatched during a thunderstorm. I was told this by an old man in Hampshire, who had been connected with the care of swans all his life. He, however, knew nothing about their singing at death. Is this opinion as to the birth of swans common? If so, probably some of your numerous correspondents will detail the form in which such belief is expressed.