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The song sparrow has made itself at home in North American cities. From parks and gardens to roadsides and residential neighbourhoods, it has adapted to living alongside people. Yet many other bird species struggle to survive as natural habitats give way to urban development.

North America has lost nearly , with habitat loss among the pressures behind the decline. As cities continue to expand, understanding why some species thrive in urban environments while others struggle could help guide efforts to protect them.

During her master’s degree at �鶹����’s, Rachel Fanelli worked with biology researchers Frances Bonier and Paul Martin to investigate the traits that might explain why. Published in , their study examined 431 North American bird species.
 

Measuring birds’ tolerance of urban environments

To study birds across such a large area, Fanelli turned to , an online platform where birdwatchers record their sightings. The platform uses millions of observations to estimate where birds live and how common they are. It’s valuable data that allowed Fanelli to compare bird populations in urban and non-urban areas.

Instead of simply classifying birds as urban tolerant or avoidant, Fanelli developed a quantitative measure of how their abundance varied between cities and all other habitats across each species’ breeding range. She then compared those results with characteristics such as body size, maximum lifespan, migratory behaviour, and breeding habitat.
 

Why some birds are better suited to cities

The study found that birds that can breed in different kinds of environments generally tolerated cities better than those with more specific habitat needs. The song sparrow fits this pattern, breeding in a wide variety of habitats, like urban parks and gardens, marshes, woodlands, grasslands, and forest edges. Species associated with boreal forests and grasslands were among the least tolerant of urban areas.

The researchers also looked at how environmental conditions, like precipitation and ambient temperature, vary across the places where a species breeds. Birds with larger breeding ranges generally tolerated cities better, but differences in precipitation across those ranges revealed something less expected. Even when two species had breeding ranges of similar size, the one breeding in places with more variation in precipitation was more likely to tolerate cities. 

“We expected birds that could live in different habitats to do better in cities,” says Dr. Bonier. “What we didn't expect was how much variation in precipitation would help explain those differences.”

One possible explanation is that precipitation shapes the availability of food, water, and nesting resources during the breeding season. Birds able to cope with varying conditions may be better prepared for changes in these resources, although the study did not directly test why this relationship exists.

The amount of food and space a bird needs may matter as much as the conditions it encounters. Smaller birds generally require fewer resources, which could help explain why they were more tolerant of cities. Species with longer lifespans were also associated with greater urban tolerance.

The black-capped chickadee weighs just 9 to 14 grams and typically remains in the same area year-round rather than migrating seasonally.

For birds that migrate, travelling to locations to establish breeding territories that can support their food, shelter, and mating needs can be energetically costly. Birds that stay in one area throughout the year can become familiar with local resources, which may help explain why they generally tolerated cities better. Species with a mix of migrating and non-migrating populations also fared better than fully migratory birds.

Closely related birds also showed similar levels of urban tolerance in about one-quarter of the species studied, suggesting that evolutionary history may play a role in how species cope with urban environments.
 

Making cities more welcoming to birds

By identifying the traits associated with lower urban tolerance, Fanelli, Dr. Bonier and Dr. Martin's research could help conservation groups recognize which species need greater attention as urban development continues. That information could guide decisions about which habitats to protect and how to make urban green spaces more welcoming to birds.

“We need to think about the birds that aren't doing well in cities,” says Fanelli. “That means creating spaces where more species can find what they need.”