Indianapolis, Indiana – Scientists have spent decades debating whether evolution would take the same path if history were somehow given a second chance. Now, new research on bird behavior suggests that, at least when it comes to the brain and its control of complex behaviors, evolution may indeed repeat itself.
The study, recently published in Nature Ecology & Evolution, was led by Kimberly Rosvall, associate professor in the Department of Biology at Indiana University, and Sara Lipshutz, former IU postdoctoral fellow and now an assistant professor at Duke University. Their findings shed important light on how some behaviors — especially aggression — seem to evolve in very similar ways across different bird species.
Focusing on birds that rely on tree cavities for nesting, Rosvall and Lipshutz found something striking. Birds that needed hard-to-find holes in trees to raise their young had evolved higher levels of aggression, particularly among females. More interesting still, these changes in behavior were matched by changes in their brains at the molecular level.
To explore this, the research team carried out field experiments with wild birds. They presented the birds with a stuffed decoy and played aggressive calls through a speaker, triggering territorial responses. This setup was repeated across hundreds of individual birds from five distinct branches of the bird family tree, including swallows, sparrows, thrushes, wood warblers, and wrens.
For each lineage, researchers compared two closely related species — one that was an obligate cavity-nester, and one that had a more flexible approach to nesting. Because obligate cavity-nesters can only reproduce after securing a hole, the scientists predicted these birds would be more aggressive. The results confirmed their expectations.
“I have been studying cavity-nesters, like tree swallows and bluebirds, for over 20 years,” Rosvall said. “We knew they fiercely defend their nesting territories, including those human-made bird boxes you might see in your local park. Now we know this ever-present competition also shapes their brain evolution.”
Digging deeper, the team analyzed gene expression in the birds’ brains. Out of more than 10,000 genes, they discovered a small set that consistently changed across all the cavity-nesting species. In every case where aggression had evolved, the same genetic adjustments showed up — even though these bird species had branched off from one another millions of years ago.
“It’s a small number of genes,” Rosvall said. “But it’s exciting because evolution did repeat itself. We knew this could happen for physical traits but not for a complex behavior like aggression.”
Beyond these repeated patterns, the team also found a larger group of genes that were associated with increased aggression in two or three branches of the bird family tree. This suggests that while nature tends to favor certain solutions, there is also room for creative variation.
As Rosvall put it, “If you asked five artists to paint the same landscape, you might expect to recognize each painting as the same scene, even if they also look a bit different. Our results are like that, except the artist is natural selection, repeatedly dialing up aggression over the last 25 million years.”
These findings demonstrate not only the predictability of evolution but also its flexibility. In a way, nature seems to work with a familiar set of tools but finds different ways to apply them depending on the situation.
Perhaps even more fascinating, the genes linked to aggression were not the usual suspects like those tied to testosterone. Instead, researchers found that the genes showing changes were associated with brain functions connected to neurodegenerative diseases in humans.
“Our results did not flag the stereotypical ‘aggression’ genes, like those related to testosterone,” Rosvall said. “Instead, we saw convergent increases in aggression linked to genes with connections to neurodegenerative disorders. This doesn’t mean aggressive birds are going to get Alzheimer’s. It just means evolution has repeatedly tweaked these genes to shift brain function and behavior. And understanding why might help us develop evolution-inspired support for people.”
Both Rosvall and Lipshutz carried out this work with the support of the National Science Foundation. Their research highlights how studies in animal behavior can have surprising implications not just for understanding the natural world, but for human health as well.
By examining natural variation, Rosvall’s lab continues to uncover how animals adapt to challenges like limited habitat and even climate change. This new study is a reminder that evolution, though full of surprises, often follows familiar patterns — even in the complex realm of behavior.
