When a population suddenly reduces in size from habitat loss, natural disaster, or other environmental changes, and its subsequent genetic variation is affected, we say it has experienced

nucleus

would be most reasonable answer tbh

fumikage isn't really a bird, and tsuyu does have teeth. tsuyu has a transformation quirk which still allows her to be partially human. otherwise we'd have a large frog running around. fumikage has teeth because again, he isn't a bird. his head was a quirk mixture if i'm correct.