Photograph by ALAN MURPHY
Photograph by ALAN MURPHY

These Birds Have Less Sex in the Suburbs

Scientists set out to study why some songbirds adapt to human encroachment and others don’t.

This story appears in the February 2018 issue of National Geographic magazine.

When land developers remove native vegetation to put in subdivisions, some songbirds do just fine. These “adapter” species find alternative places to nest and may even thrive near humans, says John Marzluff, a wildlife science professor at the University of Washington.

But other species of songbirds flee in search of undisturbed habitat, even if it means leaving a mate and losing chances to reproduce, Marzluff says. In the face of urbanization, the “avoider” species—such as the Wilson’s warbler—are known to decline.

Marzluff and his colleagues spent 12 years gathering data for one of the few studies that have been done on how urbanization affects songbird species’ dispersal. The researchers identified three types of sites: forest reserves, existing subdivisions, and “changing sites” where forest was being turned into subdivision. There they caught and banded avoider and adapter birds—nearly 3,000 in all—then tracked where the birds went and whether they fledged broods.

The species studied typically stay with one mate in one area. But when development removed the low plants where avoiders like to nest, researchers saw the birds relocate and “divorce,” or not reunite with, their prior mate. When finding a new partner and new territory disrupts a breeding season, avoiders “often fail to produce young,” Marzluff says. “For a bird that lives five or six years, that’s a big hit.”

To thrive and multiply, avoider birds will need native habitat reserves. But many species of adaptable birds “do very well around us,” Marzluff says. “It’s important for people to realize that we can do a lot in our yards and neighborhoods to foster birds.”