If you ever felt at least metaphorically like you wanted to kill your brother or sister, your sibling is lucky you're not a booby. A Nazca booby, a Galápagos Island seabird, is eager and able to kill a sibling in the nest.

A new study links the murderous behavior to high levels of testosterone and other male hormones found in the hatchlings.

The elevated levels of male hormones, called androgens, increase aggression in both male and female chicks and prepare the birds to fight to the death as soon as they hatch, said David J. Anderson, professor of biology at Wake Forest University. "The older of two Nazca booby hatchlings unconditionally attacks and ejects the younger from the nest within days of hatching."

The reason for this sibling spat: The parents of this species find it hard to raise more than one, so by wiping out the younger sibling, the older chick stands a better chance of surviving.

The bullying doesn't stop in the nest. Surviving chicks frequently seek out nestlings in their colony, and during those visits they often bite and push around the defenseless youngsters, the researchers found.

Blood samples were taken from chicks within 24 hours of hatching. In 15 nests with two eggs, blood samples were taken from both hatchlings. Samples were also taken from 15 hatchlings in one-egg nests. Some Nazca booby nestlings experience a one-two hormonal punch, raising their aggression hormones even higher when they actually have a nest mate. The nestlings that fight siblings become bigger bullies as adults than the Nazca booby nestlings who never fight.

"The hormones that are part of this epic battle early in life seem to permanently change some aspects of their social personality," Anderson said.

The finding is detailed in the June 18 edition of the online journal PLoS ONE. Much of the field work was done by graduate student Martina Müller.

Nazca booby chicks have aggression-related hormone levels three times as high as their less aggressive cousins, the blue-footed boobies. Blue-footed boobies do not have the same lethal fights right after hatching and do not go on to bully their fellow birds as adults.

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