While the story of Leda, the swan, and the giant human eggs is probably the best known version of Helen's birth, it's not the only one, nor is it likely the earliest one. The good news, for simplicity's sake, is pretty much every version features people turning into birds and babies coming out of giant eggs.
The mostly lost epic poem Cypria records Helen's mother was actually Nemesis, the goddess of righteous retribution. Zeus wanted to get with her, but she was like "nah" and turned into a goose and flew away, but Zeus was like "yes, actually" and also turned into a goose and chased her and, well, you know. Zeus gets what he wants. Nemesis laid an egg that was then transferred to Leda, who hatched it and raised the divine baby she found inside, like the highest-stakes Kinder Surprise of all time.
Because of the similarity of Helen's Greek name (Helene) to the word for "moon" (Selene), Helen was often associated with the moon. In fact, one follower of Pythagoras (yes, the triangle guy) suggested Helen was in fact born on a moon colony but her egg dropped to Earth where she was hatched. While this would explain her unearthly beauty, the argument against this is, of course, moon people are fifteen times larger than Earth people.
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