Angry birds: Why some females are more aggressive

“Get off my lawn!” That phrase, often joked about as a grumpy old man’s warning to neighborhood kids, might just as well apply to a fiercely protective bird.

This is especially the case when it comes to birds that depend on rare, hard-to-find real estate: tree holes.

Not all birds react with this much attitude. But researchers have found that certain female birds – those that can only nest in cavities – show a striking level of aggression when defending their homes.

These aren’t just random outbursts. They are calculated, defensive behaviors tied to how and where they can reproduce.

Why cavity-nesting birds get aggressive

The study was led by experts at Duke University, Indiana University, and several international institutions.

The team focused on a group known as obligate secondary cavity nesters. These are birds that must find existing holes in trees, fence posts, or rocky crevices to raise their young. They can’t dig their own cavities, and they can’t simply build nests anywhere.

“They can’t excavate that cavity themselves, and they can’t just build a nest anywhere,” explained Sara Lipshutz, assistant professor of biology at Duke. “They have to find a hole in a tree, and this is the only way they can reproduce.”

This makes nesting spots incredibly valuable – and worth fighting for. These birds don’t just chase away threats. They screech, flap, dive, and even peck to protect what little they have.

Testing bird defenses in the field

To see how common this behavior is, the team studied five families of birds – swallows, wood warblers, sparrows, thrushes, and wrens. Within each family, they compared two species: one that nests in cavities and one that doesn’t.

The setup was clever. The researchers placed lifelike decoys and broadcast bird calls via Bluetooth speakers near nests. Then, they sat back and observed how the resident birds reacted.

The differences were obvious. Most of the obligate cavity nesters were far more aggressive than their more flexible cousins. They didn’t just respond – they attacked.

Female birds especially showed a stronger reaction, likely because they stood to lose more if their nesting space was taken over. “It was a really striking behavioral pattern,” said Lipshutz.

Aggression isn’t all in the family

You might expect some of this aggressiveness to be genetic – passed down through bird families. But that’s not what the researchers found. Nesting strategy was a far better predictor of aggression than bird family.

There were two exceptions: sparrows were generally mellow, while wrens were always feisty.

“The pressure to compete led to higher aggressiveness. And this was especially strong for the females,” noted Kimberly Rosvall, a Duke alumna whose lab at Indiana University contributed to the project.

Testosterone isn’t to blame

Hormones like testosterone often play a role in animal behavior, so the team checked if the more aggressive birds had higher levels of it. Surprisingly, they didn’t.

Female cavity-nesters were not pumping out extra testosterone, at least not in a way that could explain the fierceness.

This left the researchers wondering if something deeper – like gene activity – might be behind the behavior.

A genetic puzzle of bird aggression

To find clues, the scientists looked at how genes were expressed across the different bird species. Were the same genes being flipped “on” in aggressive birds?

The answer was murky. Yes, there were some patterns in how certain genes were activated in cavity-nesting birds, but there was no one-size-fits-all answer. The genes tied to aggression varied between species, and the usual suspects weren’t showing up.

“We always want to find the genes, a handful of genes that make a lot of sense, previously known genes that are associated with aggression and maybe have even been experimentally manipulated to impact aggression,” said Lipshutz. “And we didn’t find any of those usual suspects.”

Different genes, same fierce results

Rather than finding one or two major aggression genes, the researchers believe there are probably hundreds of genes involved. Each species seems to use a different combination of them. That might sound like a frustrating result, but to the team, it’s a fascinating one.

“There are probably several hundred genes associated with aggression and they’re probably all working together in subtle ways. Those combinations are not the same for each species, which is really interesting from an evolutionary perspective,” said Lipshutz.

“It shows that there are independent ways to get to the same behavioral outcome. As our team likes to say, ‘There are many possible routes to build an aggressive bird.’”

The full study was published in the journal Nature Ecology & Evolution.

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