The Common Cuckoo
Cuckoo birds are among the biggest jerks in the entire animal kingdom. They want to have babies, but they're not into the whole parenthood thing. So much work, am I right? So they lay their eggs in the nests of other species of birds and let them deal with their demon offspring. These duped parents often lose their own kids because cuckoo babies are so big and aggressive (I mean, look at the picture!). And get this: if a bird catches on and tries to eject or destroy the cuckoo egg, the adult cuckoos will tear its nest apart and break all the eggs! This insane behavior is called the "Mafia hypothesis." Cuckoos are such unapologetic bullies, you almost have to love them for it.