As California’s cities expand and encroach upon natural landscapes, the state’s wildlife is adapting in fascinating ways. One of the most compelling examples of urban ecology at work is the story of the dark-eyed junco (Junco hyemalis), a small songbird traditionally found in mountainous forests, now thriving in urban environments like San Diego and Los Angeles. These birds are demonstrating the rapid adaptability of species when faced with the unique pressures of urban life, providing a living case study of how evolution unfolds in real-time in human-modified habitats.

Traditionally, dark-eyed juncos were migratory birds, spending summers breeding in cooler high-altitude forests and wintering at lower elevations. However, in the early 1980s, a population of juncos was observed settling on the campus of the University of California, San Diego (UCSD), foregoing their usual migratory behavior. Researchers soon noticed that these urban juncos were behaving differently from their forest-dwelling counterparts. Similarly, at the University of California, Los Angeles (UCLA), a junco population has been thriving for decades, with numbers reaching approximately 300. This long-term success has provided the Yeh Lab at UCLA with a unique opportunity to study how urban environments influence the evolution and behavior of these adaptable songbirds. Their research sheds light on how juncos have adjusted to city life, offering broader insights into wildlife resilience in human-altered habitats.

In their natural forest environments, juncos breed in response to the changing seasons, triggered by increasing daylight hours and rising temperatures. But in urban areas like those around UCSD and UCLA, where food is plentiful year-round, juncos have begun breeding earlier than normal and throughout the year. They build nests higher off the ground, often on artificial structures, and have increased the number of clutches per breeding season. The availability of artificial light, abundant food from human sources, and fewer natural predators in the city all play roles in these behavioral shifts. According to a study published in Behavioral Ecology, these changes in breeding patterns are a direct result of the birds’ adaptation to urban conditions.

One of the most striking adaptations among urban juncos is their behavioral shift. Unlike their shy mountain counterparts, urban juncos are much more tolerant of human presence. This is not only a matter of convenience; it’s a survival mechanism. In the city, humans are not a threat, and urban birds need to capitalize on the resources provided by their proximity to people.

“Juncos are important because they’re doing something that we want all birds to do: be persistent in cities, be able to adapt, multiply and have — assumingly — a positive impact on the environment,” said Joey Di Liberto, a doctoral student in the UCLA Yeh Lab. “They’re a model of how we want, or hopefully can have, animals react to strong environmental change.” 

A study conducted by the University of Virginia showed that urban juncos are less aggressive and territorial than rural juncos. In fact, their increased tolerance for close encounters with humans is a behavioral trait that likely enhances their chances of survival in dense urban environments. This is a perfect example of natural selection at work in an urban setting, where the traits that help a species succeed in nature shift to those that help them succeed in human environments.

Beyond behavior, there are physical differences between urban and rural populations of dark-eyed juncos. Urban juncos, for example, have been found to have darker plumage than their rural relatives. This difference in appearance may be linked to the heat-absorbing properties of darker feathers, which could help the birds cope with the higher temperatures often found in urban areas. Additionally, darker plumage may provide a level of camouflage in the concrete jungle, a phenomenon that has been observed in other urban-dwelling bird species as well.

A study published in the Journal of Avian Biology explored how urban juncos are more sedentary compared to their migratory relatives. This sedentary lifestyle has significant physiological implications, particularly in energy allocation. Since urban juncos do not need to expend as much energy on long migratory flights, they can invest more in reproduction and adapting to their environment, such as becoming more accustomed to noise and artificial light.

The dark-eyed junco is just one example of the broader phenomenon of urban ecology, where species adjust to the challenges and opportunities posed by city life. In California, this phenomenon extends beyond birds. Coyotes, for example, have become fixtures in cities like Los Angeles, adapting to scavenge food from human waste. Coastal animals like sea lions and pelicans have also made urban waterfronts their home, thriving amid the bustle of human activity. (California Curated explored how the Western Fence Lizard has swiftly adapted to life in an urbanized environment.)

These adaptations reflect a larger trend—urban areas are becoming new ecological niches, where animals that can adjust to the artificial conditions of city life will survive and potentially thrive. However, while species like the junco illustrate the resilience of wildlife, they also raise questions about the long-term impacts of urbanization on biodiversity.

The study of urban ecology and evolution has gained increasing attention in recent years. Researchers, such as those at UCSD and UCLA, are beginning to understand how cities act as ecosystems of their own, offering new challenges and opportunities for wildlife. Urban environments are dynamic systems that rapidly alter the selective pressures on species, leading to changes in behavior, physiology, and even genetics over relatively short time spans.

In his book Darwin Comes to Town, Dutch evolutionary biologist Menno Schilthuizen highlights the junco as an exemplar of rapid evolution in urban settings. But other sources, such as the work published in Trends in Ecology & Evolution, suggest that urbanization can lead to both positive and negative outcomes for wildlife. While some species like juncos flourish, others may struggle to survive amid shrinking habitats, pollution, and human interference.

The dark-eyed junco’s urban adaptation offers a hopeful but cautionary tale. On one hand, it demonstrates the incredible capacity of wildlife to adjust and survive in the face of human expansion. On the other, it serves as a reminder of the pressure we place on natural ecosystems as our cities grow. As researchers continue to study urban ecology in California and beyond, the junco remains a powerful symbol of the way life endures, adapts, and evolves in response to the ever-changing world around it.

For those of us who live in cities, the juncos flitting around our parks and campuses offer a rare opportunity to witness evolution in action—a reminder that nature is not something separate from our urban spaces, but something woven into the very fabric of our lives.