If that headline doesn't drive traffic to blogfish, I don't know what will. Young Nazca booby chicks get very aggressive; the firstborn kills the younger chick to increase its own chances of survival. Researchers at Wake Forest University* found that firstborn chicks -- both male and female -- get spikes of androgens that increase their aggression not only when they're babies but also throughout their lives. This is not the case for the Nazca's cousins, the blue-footed boobies, who spend their time peacefully comparing their feet.
* Cheers to Muller et al for publishing in PLoS so we can all read the original paper.