This leaves US/ME/THEM/WHOMEVER for theories. It means « we just have theories ramaining. » The scientists still argue ABOUT them. (About the theories.) How was that possible THAT an animal without feathers becomes a bird? “That” introduces the noun clause “an animal without feathers becomes a bird?”. Birds often feed ON the rubbish left by people. It means that birds eat the rubbish that people left. I hope that helps.