Wake Forest University researchers led by Professor David Anderson have linked the murderous behavior of the Galapagos Island birds to high levels of testosterone and other male hormones found in hatchlings. The male hormones increase aggression in both male and female chicks, preparing the birds to fight to the death as soon as they hatch.

"The older of two Nazca booby hatchlings unconditionally attacks and ejects the younger from the nest within days of hatching," Anderson said. Because Nazca boobies have difficulty raising more than one chick, it is important for the older chick to vanquish the younger one to increase its chances of survival, he said.

The researchers also discovered the high hormonal levels cause surviving chicks to behave like bullies even after they grow up.

Blood samples taken from Nazca booby chicks within 24 hours of hatching were found to have aggression-related hormone levels three times as high as their less aggressive cousins, the blue-footed boobies.

The new findings are reported in the online journal PLoS One.