In human relationships, sibling rivalry usually takes the form of stealing toys or setting up imaginary lines no one can cross. Parents might not want to be dragged into these scuffles, but they'll usually referee the proceedings if they devolve into actual violence. Not so with our raptor friends, who may let one baby actually kill another.
Granted: I hate birds. So perhaps the behavior isn't as unduly harsh as I claim. It's commonly seen in eagles (both golden and black, although it's not unknown in bald), and it's usually a matter of the firstborn chick fighting for -- or even just receiving -- the most food and resources from mom [source: Ehrlich et al.]. However, sometimes the bigger baby actually will peck to death the little one, as Mother Bird casually watches [source: National Geographic]. Because birds are nuts.
Or, I will grudgingly admit, because Mother Bird knows just as well as Baby Bird that there's just not enough resources to go around, and the strongest will survive. As the youngest (and absurdly smallest) in a family of all (burly) males, I for one am very grateful this parenting trait never got to surprise me firsthand.