In cities across the globe, animals are becoming bolder, more aggressive, and quicker to explore—and a sweeping analysis of 133 species suggests this transformation is happening everywhere urbanization takes hold. Researchers at Lewis & Clark College, CEFE-CNRS in Montpellier, and North Dakota State University combined data from 80 existing studies spanning 28 countries to reveal what may be one of the most consistent behavioral shifts in the animal kingdom: urban life rewires how wild creatures behave.
The finding matters because it suggests a fundamental mismatch is unfolding between humans and wildlife. As cities sprawl and animals adapt, the creatures we share our urban spaces with are becoming less cautious, less afraid of human presence, and more willing to take risks. Dr. Tracy Burkhard, the study's first author and an assistant professor of biology at Lewis & Clark College, put it plainly: "We found that no matter where you are in the world, urbanization is changing behavior in consistent, predictable ways. The strongest result was that animals seemed to be more risk positive. They're more bold."
The effect appears strongest in birds, which make up over 70 percent of the research examined. Urban bird populations showed pronounced increases in boldness, aggression, and exploratory behavior compared to rural flocks. But the pattern extends beyond the usual suspects—the rats, gulls, and pigeons we expect in cities. The same behavioral shifts are appearing in species traditionally associated with rural habitats: whitethroats, yellow hammers, and redpolls are all becoming bolder as they adapt to urban environments. These findings were published in the Journal of Animal Ecology.
The worry is clear: if animals lose their wariness of humans, contact between people and wildlife will intensify—potentially in ways that harm both. "If animals are more risk-taking and they're less averse to human presence, we're going to be coming into contact with wildlife a lot more in certain areas, and that is potentially bad for both us and wildlife," Burkhard cautioned. The risks include increased human-wildlife conflict and a heightened risk of zoonotic disease transmission, where pathogens jump from animals to humans.
The research also exposes a significant gap in what we understand about urbanization's effects on animal behavior. Insects, amphibians, and reptiles account for just 10 percent of the data analyzed, leaving huge swaths of the animal kingdom understudied. Dr. Anne Charmantier, a co-author and research director at CNRS University of Montpellier, emphasized this imbalance: "The research effort is very imbalanced across taxa; in particular, birds are much more commonly studied than amphibians, reptiles or insects. The lack of data in some animal classes limits some of our conclusions and should be seen as an encouragement to study all organisms living in cities."
As urbanization accelerates worldwide, the findings point to an urgent need for cities to plan with animal behavior in mind. Creating connected greenspaces that allow urban animal populations to maintain genetic diversity and connectivity may help mitigate conflict. The study suggests that how cities grow—and how they design space for wildlife—matters profoundly for the creatures who share them with us. Understanding these behavioral shifts is the first step toward coexistence in an increasingly urban world.