For birds, siblinghood can be a matter of life or death

By Nell Greenfieldboyce

Published May 1, 2024 at 5:00 AM EDT

The Science of Siblings is a series exploring the ways our siblings can influence us, from our money and our mental health, all the way down to our very molecules. We'll be sharing these stories over several weeks.

Human sibling relationships can get pretty complicated, but in the world of birds, siblings can be so impactful that their existence can mean the difference between life and death.

That's because avian nest-mates sometimes forge close, collaborative relationships that last their entire lives.

Other times, after an egg hatches, a baby bird might immediately get killed by its older brother or sister.

That depressing alternative is what fascinated David Anderson of Wake Forest University when he first noticed it a few decades ago. Back then, he was working as a research assistant in the Galápagos Islands, and he'd often pass by nests on the ground made by large white and black seabirds called Nazca boobies.

He'd invariably see nests with two chicks inside, a big chick and a little chick — but then the little one would disappear.

"Always. And I would frequently find it a meter, half-meter away from the nest site, just dead on the ground, apparently unrescued by the parents," says Anderson.

These deaths turned out to be siblicide, the murder of one sibling by another. And in this species, the killing was the kind of routine practice that researchers now call "obligate" siblicide, to distinguish it from the kind of less-frequent intra-nest murder that can occur in some species when food is scarce.

Though avian sibling-on-sibling violence can be dramatic and attention-getting, it's not part of life for most bird species. "It's common for sibling competition to happen in a nest of bird babies," says Anderson. "It is relatively rare for them to actually attack each other."

Nature's backup plan

Among Nazca boobies, the violence seems to exist because it serves a larger purpose, according to research done by Anderson and his colleagues.

He explains that for some reason, this species is terrible at hatching eggs. "They can only hatch about 60% of their first eggs, which is appallingly bad," says Anderson.

So a few days after laying an egg, the mother lays a second one, as a kind of insurance policy in case the first egg fails. But if the first egg hatches successfully, then the backup isn't needed and it would be too much work to feed a second hungry mouth. That's why nature — in the form of the older sibling — simply kills the second chick off, as soon as it hatches.

Nazca boobies lay one or two eggs. If two eggs hatch, the larger chick pushes the smaller from the nest, resulting in its death. Here, the larger sibling is 6 days old; the smaller one is 1 day old.

"The big chick will suddenly start pushing on the little chick, kind of out of nowhere," says Anderson. "Usually, the situation is that the little chick has reached up for food from the parent, so it has exposed its neck. And then the big chick hits the neck and digs in with its ridiculous little hind feet and pushes the little chick a little bit."

The bigger chick keeps doing that until its younger sibling is out of the nest. "All of this happens in about two minutes," he says. "If you're not there in those two minutes, you don't see it."

Once the little chick is out of the nest, its mother won't retrieve it or feed it, and it's doomed.

A united front

Clearly, sibling relationships can be deadly serious in birds.

In some species, siblings will fight together as a united front to help each other — even if doing so means taking enormous risks.

Acorn woodpeckers in western North America, for example, have extraordinarily complex social lives. They commonly live in cooperative groups, and siblings frequently share mates as well as child care duties.

Acorn woodpeckers perch in a dead tree in the Angeles National Forest. These birds can form close, lifelong bonds with their siblings.

"You may have, for instance, a pair of brothers who then form a group with a mother and her two daughters. And they will nest together in the same nest and raise all their offspring together," says ecologist Natasha Hagemeyer, who has studied these birds in California.

Her research has found that relatives will often help each other to set out from their family's territory and establish new homes.

One particular group of five sisters needed to find their own new territories before they could start breeding, she recalls.

So when attractive territories became available, these sisters repeatedly teamed up to fight off other birds that wanted to occupy the same choice spots. These fights can be brutal, she says.

"They involve birds chasing each other, screaming, displaying and also grappling in midair, to the point where sometimes they will hit the ground and still be fighting," says Hagemeyer. "I've seen birds get killed during these fights. So it is very risky."

The five sisters, however, were unstoppable as they worked as a united front. After one sister had been helped to get a territory in one power struggle, says Hagemeyer, she'd later come to assist her other sisters in a subsequent battle.

In the end, Hagemeyer says, every one of these loyal sisters ended up in a situation where they could each start raising their own nests full of babies — the next generation of bird siblings that, in the future, might help each other survive.