As cities gobble up wilderness, some birds have learned to live alongside skyscrapers, traffic and noise, and large numbers of humans.
A UCLA-led team of biologists wondered if these city-dwelling birds share common characteristics throughout the world that help them survive. In an article published in current biology, reveal the answer: urban bird species tend to be smaller and less territorial and have greater ability to fly long distances. They also tend to have broader dietary and habitat niches, lay more eggs at one time, have longer lifespans, and live at a wider range of elevations than other species.
Several factors moderate the importance of those traits in how birds adapt to urban life, and the importance of those traits varies in predictable ways across the planet.
The only trait that did not appear to be globally consistent among city-dwelling birds, the researchers found, was the shape of their beaks.
By 2030, the paper notes, the amount of urban land cover worldwide will have grown by 1.2 million square kilometers (or more than 463,000 square miles) since 2000, nearly tripling in a 30-year period. That increase in urban space would be greater than the land area of California and Texas combined.
The authors write that a dramatic loss of biodiversity will accompany such urbanization unless there are practical plans to preserve it.
“Identifying the traits that help wildlife adapt or even thrive in cities can help urban planners enhance biodiversity, for example by increasing green spaces, planting more and taller trees, building more varied potential habitats or reducing housing density,” said the paper’s lead author. , Monte Neate-Clegg, UCLA postdoctoral researcher. “It can also help conservation biologists identify which types of species are most threatened by urbanization.”
Neate-Clegg and Morgan Tingley, a UCLA associate professor of ecology and evolutionary biology, combined data including records of more than 125 million individual bird sightings from the eBird public science project to calculate an “urban association index” that describes what how closely is each species associated with living in urban environments.
They applied the measure to 3,768 bird species, around 35% of all bird species, in 137 cities on six continents.
The index takes into account factors such as the physical characteristics of the birds themselves and the geographical, population and landscape characteristics of the cities.
“Many of the most common urban birds globally are very familiar to us here in the US, including the house sparrow, barn swallow, osprey and peregrine falcon,” Tingley said. “Though, interestingly, the species with the strongest associations with urban areas are actually three species of parakeet and one tanager from South America. Plus, of course, the feral pigeon.”
Interestingly, some of the species with the highest scores on the index, meaning they were most closely associated with urban life, were not native to their regions, but those species represented less than 4% of the data set, which suggests that invasive species might not have as great an advantage as logic would suggest. Bird families with high average scores on the index, indicating that many species within that family were common in cities, included starlings, swifts, swallows, parrots, orioles, and blackbirds.
Traits such as smaller body size, less territoriality, increased ability to fly longer distances—what scientists call “dispersal ability,” broader dietary and habitat niches, larger clutches—tend to make it easier for birds of the city find food. and suitable places to nest, and to raise surviving young.
Bird species that generally build nests on the ground probably don’t live in cities, for fairly obvious reasons.
“In a city like Los Angeles, for example, the American crow is a cosmopolitan species whose broad diet, tree-nesting habits, and long life span favor life in the concrete jungle,” Neate-Clegg said. “In contrast, canyon wrens are highly territorial insectivores with low dispersal ability that avoid cities and stick to steep, rocky terrain.”
But the geographic properties of cities, especially their latitude, moderated the importance of those features. For example, a broad diet was more important in temperate cities like New York, while habitat generalists were more frequent in tropical cities like Bogotá, Colombia. The population size of cities and the surrounding terrain also played a role: Anchorage, Alaska, with its small population and abundant natural environment, is home to large birds such as bald eagles that feed primarily on fish from rivers and lakes. Meanwhile, smaller birds that don’t require as much raw land and can adapt to eating a variety of foods to survive would be more likely to inhabit densely populated metropolises like Bangkok.
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