If, as DarkLady suggests, phoenicism hits at puberty, shouldn't David have been born in a shell? Roger wouldn't have known; those were the days when the father-to-be paced in the waiting room.
And MysticScribe's mention of gods siring offspring with humans made me think of Leda and the Swan (and Yeats's fantastic poem). Like Magnus, I have always assumed that Laura has existed for centuries, but maybe she's a fairly recent creation - an all-American (well, half-American) phoenix, spawned in 1667, and her mother was a prim and proper Puritan maiden before bedding a bird.