Unit cost and quality.

No, champs have a higher growth. But champs aren't it... once the birds hit with thunder, that's all. That's why I would raise the price for the birds...

Well, more grows taking into account the Barbarian building (forgot the name) 50% to growth=) if you raise the price of the birds, no one will build them; they'll say why should I overpay for a unit with a powerful one-time attack (because the birds might not hit a second time) if there are monsters (strong health and strong attack), I'll survive their speed somehow, I'll find acceleration) Moreover, birds don't hit everyone with lightning, and from some units they'll actually get it back (they might not be very effective against Chaos and Nature), by the way, the birds' lightning effectively doesn't work on vampires either, but instead they take full damage from the bird, guess why :D