Sibling Desperado: Doomed booby chick turns relentlessly violent

The first known case among nonhuman vertebrates of so-called desperado aggression–relentless attacks against an overwhelming force–may come from the underling chick in nests of brown boobies.

SIBLICIDE. The older, larger chick pushes its younger sibling toward the edge of a brown booby nest. Later, the older chick expelled the younger one, and the parents didn’t rescue it. J.L. Osorno

An unusual experiment that tucked junior chicks into the nests of a related species let the youngsters live long enough to show their stuff, says Hugh Drummond of the Universidad Nacional Autónoma de México in Mexico City. These chicks ferociously attacked their older foster sibs, and almost half of the relocated chicks became “uncontrollably aggressive,” Drummond and his colleagues report in an upcoming Behavioral Ecology and Sociobiology.

The work could bring more respect for the influence of the underdogs in driving the violence at the top of a hierarchy, Drummond predicts. “The message is that [at the top] you can’t afford to be generous if the little guy is going to turn into a whirlwind of violence,” he says.

The Pacific seabirds called brown boobies lay two eggs but hardly ever fledge more than one chick. In nests where both eggs hatch, the older one pecks and pushes the younger one and almost always eventually expels it from the nest. However compared with related species, brown booby adults aren’t unusually aggressive.

Those younger chicks have been a challenge to study in the nest. “They tend to be either underneath the parent or they’re being beaten up on by a sibling that’s trying to kill them,” says Drummond.

To observe the underchicks, he and his colleagues went to San Pedro Mrtir in the Sea of Cortez, one of the rare places where brown boobies nest in sync with blue-footed boobies. In the latter species, the chicks work out a less-violent hierarchy and both often survive when food is adequate. Drummond’s team observed nine brown booby underlings and nine blue-footed underlings that the researchers had transferred into foster nests. Each underling joined a blue-footed chick, approximately 5 days older, in the new nest.

As underlings to a blue-footed chick, five of the brown boobies delivered about the same number of pecks and shoves as the underling blue-footed sibs did. Four of the brown booby chicks, however, turned extraordinarily violent–making 100 to 700 attacks per hour on the resident chick, which was nearly twice their size. In one case, a youngster actually drove the older chick out of the nest. The researchers stopped the experiment after 18 days to save the blue-footed booby chicks.

Unlike blue-footed booby underling chicks, the browns “didn’t learn to be submissive,” says Drummond.

The ferocity of the brown booby youngsters strikes Drummond as an example of the desperado aggression that a theorist predicted decades ago. “Since they’re doomed, they’re prepared to go for bust,” Drummond says.

That sounds plausible, agrees bird ecologist Scott Forbes of the University of Winnipeg in Manitoba. “There’s a myth that families ought to be harmonious, but we’re finding that conflict is a large part, a very natural component, of family relationships.”


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Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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