It’s no secret that birds are struggling. North America has lost billions of them since the 1970s and today more than one-third of bird species are considered under threat. While efforts to stem the tide tend to target endangered and rare species, new research shows that the bulk of losses are coming from the most common birds, such as Rufous Hummingbirds that flit around home feeders and Barn Swallows that swoop over farm fields.
“We think of them as such staple species, like they’ll always be there,” says Gates Dupont, an ecologist at Princeton University. But when Dupont and biologist Andy Dobson looked at nearly six decades of data on 244 bird species, they found that abundant species saw the steepest drops in their numbers.
Their findings, published July 30 in Science Advances, show that just 5 percent of species make up 80 percent of the total declines in avian abundance, according to data from the U.S. Geological Survey’s North American Breeding Bird Survey. At the same time, some rare species have been increasing their populations.
Crucially, the study also pinpoints the main driver for the declines: Human-caused destruction of habitat, including forests and grasslands. As development increases, there’s less space for birds to live. Because common species are closer to their “carrying capacity”—meaning they’re nearly maxing out the number of birds that their ecosystem can support—any reduction in available habitat means fewer birds can survive. Meanwhile, land-use changes don’t affect rare species as dramatically because their populations are further from their carrying capacity, the study suggests.
“If the landscape is completely full of birds and you take away some of that natural landscape, you’re going to lose birds,” says Nicole Michel, director of quantitative science at Audubon, who was not part of the study.
Dupont and Dobson’s findings are the latest in a growing body of research on common birds declining. A study in May found that bird populations fell the most in areas where they were most abundant, per eBird data. Similar trends have been observed in European birds. Insects also show the same patterns: Species with the highest numbers have experienced the greatest losses in recent years, according to a 2023 study.
It’s a phenomenon Michel has seen herself. Audubon’s Christmas Bird Count, which has enlisted volunteers to gather data on North American bird populations since 1900, has recorded a collapse in the number of Red-winged Blackbirds, Common Grackles, Mallards, and American Robins, among others. “If these declines continue, these formerly common birds may become rare,” Michel says.
Losing common birds “impacts us all,” Michel says, since they provide the bulk of benefits to ecosystems: They eat insects and small rodents, pollinate plants, disperse seeds, and help cycle nutrients in soil. Meanwhile, rare species tend to develop niche roles, like the Lucifer Hummingbird, with its beak curved perfectly to fit inside flowering agave plants. That makes it harder for rare birds to fill in for the services lost by common ones.
“If these declines continue, these formerly common birds may become rare.”
The downward trend in common birds also raises broader concerns about the health of the environment. “Birds are excellent indicators,” Michel says. “When there’s trouble with birds, it’s an indicator that there’s trouble in the larger ecosystem.”
Knowing exactly which species are suffering the most—and why—is critical to figuring out how to take action. In this case, the findings suggest that helping common birds will require a bigger push to combat habitat loss, the authors say. “We need conservation efforts that meet the scale that humanity is changing the environment,” says Dupont.
A great place to start, Dupont says, is by improving existing forests to accommodate more wildlife. “The quality of the forests is not what it needs to be,” he says. Many forests in the United States today are homogenous—a consequence of reforestation efforts that followed massive clear-cutting for logging and agriculture. Planting more native species and managing forests to cultivate a wider range of tree ages could go a long way to help more birds find food and shelter.
The conversion of forests and grasslands to farmland is harder to tackle. But one solution is working with ranchers and other landowners to implement bird-friendly practices, since the majority of grassland birds live on privately owned land. Rotational grazing, for example, can help increase biodiversity and restore vital habitat.
History shows that policy action can help turn trends around for birds in trouble. Dupont points to the ban on the harmful pesticide DDT in 1972 that led to major comebacks for the Bald Eagle, Osprey, and other birds. “We identified a problem, we addressed it through policy, and the birds bounced back,” he says. “That provides a lot of hope.”
