As near as I can remember, we didn't plant a single sunflower out back this year:

That's all from birdseed or seeds from last year's flowers. You can see that they're all the smaller, wild type of sunflower. I think what happens is that the hybrids you palnt - those giant seedheaded ones - rever to their wild type ancestor if you let 'em go to seed. (Is that right, Dano?)