Urban development is booming, and with 9.7 billion people expected to populate the world by 2050, it’s no wonder. But the human need to build, build, build is altering a more feathery and flighty world, according to researchers who are studying how urbanization and suburban sprawl affect native bird populations.

A new study in the journal PLoS One is among the latest to focus on this issue, and it reached a startling conclusion: Development is forcing some typically monogamous birds to “divorce” their mates, flee their once-forested homes and start over in a new landscape — a transition that can cause them to lose half their breeding years.

That finding grew out of a decade-long project led by University of Washington wildlife science professor John Marzluff, whose team monitored six species of songbirds in three landscapes of the Seattle area: forested preserves, already developed suburban neighborhoods and neighborhoods transitioning from forest to subdivision. They banded birds’ legs and mapped their travels, keeping tabs on nesting, mating and breakups.

“We’ve been studying this issue from many different angles over the years,” Marzluff said in an interview. “Basically what we’re trying to figure out is why the community of birds changed so much with urbanization.”

Two of the species — the Pacific wren and Swainson’s thrush — were dubbed “avoiders” because of their dependence on ground shrubs and brush and their shyness around humans, characteristics that make them unable to adapt well to urban encroachment and the landscaped lawns that come with it. They were divorcing one another at a consistent rate and leaving in search of more forested habitat, the researchers found. Four other species, considered “adapters,” went about their business next to their new human neighbors; some even thrived in their newly developed areas.

Other studies have found similar effects of city life on bird development and behavior. Researchers at Ludwig Maximilian University and the Max Planck Institute of Ornithology studied great tits — a bird scientists call a synanthrope, or a species that benefits from living in close proximity to humans — and found that they also appear to suffer reproductively, yet in a slightly different, though still disruptive, way. While great tits bred earlier in urban areas, their eggs were consistently smaller and their nestlings weighed less than great tits born in the country.

Another study on great tits, carried out by researchers at Lund University in Sweden, determined that city birds had a higher propensity for dying young than their rural counterparts.

Yet birds can also benefit from urbanization. After marveling at the ability of Barbados bullfinches to snatch food from humans’ plates, Jean-Nicolas Audet, a doctoral candidate at Canada’s McGill University, was inspired to study them. Audet compared rural and urban bullfinches on the island and found the city birds had better cognitive abilities and problem-solving skills. They were also less skittish around humans and were overall more confident – perhaps because of the daily commotion in their urban environment.

Some initiatives, including the U.S. Fish & Wildlife Service’s Urban Bird Treaty, are aimed at conserving bird habitats within cities and reducing hazards to urban and suburban birds. But given that development isn’t going anywhere, researchers say understanding how cityscapes influence birds is essential to protecting those that live among us.

Marzluff, for his part, doesn’t think urbanization is a net negative for birds. In fact, he said, the diversity of birds within subdivisions is usually higher than in the reserves created for birds or in city centers.

“There’s a strong selective force on the birds,” in urban environments, said Marzluff. “I think they adapt quite readily and rapidly to our presence, but there’s a whole group that can’t — that’s the avoiders – and, over the long haul, I think they’re going to fade out and you’re not going to have those birds in those developed areas. They’re going to absolutely require us to reserve some habitat for them. But, fortunately, at least in North America, that’s a relative minority of birds compared to all the ones that can and do live among us.”

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