North America's birds are disappearing from the skies at a rate that's shocking even to ornithologists. Since the 1970s, the continent has lost 3 billion birds, nearly 30% of the total, and even common birds such as sparrows and blackbirds are in decline, U.S. and Canadian researchers report this week online in Science. "It's staggering," says first author Ken Rosenberg, a conservation scientist at the Cornell University Laboratory of Ornithology. The findings raise fears that some familiar species could go the way of the passenger pigeon, a species once so abundant that its extinction in the early 1900s seemed unthinkable.
The results, from the most comprehensive inventory ever done of North American birds, point to ecosystems in disarray because of habitat loss and other factors that have yet to be pinned down, researchers say. Yet ecologist Paul Ehrlich at Stanford University in Palo Alto, California, who has been warning about shrinking plant and animal populations for decades, sees some hope in this new jolt of bad news: "It might stir needed action in light of the public interest in our feathered friends."
In past decades, Ehrlich and others have documented the decline of particular bird groups, including migratory songbirds. But 5 years ago, Rosenberg; Peter Marra, a conservation biologist now at Georgetown University in Washington, D.C.; and their colleagues decided to take a broader look at what is happening in North America's skies. They first turned to the North American Breeding Bird Survey, an annual spring census carried out by volunteers across Canada and the United States, which has amassed decades of data about 420 bird species. The team also drew on the Audubon Christmas Bird Count for data on about 55 species found in boreal forests and the Arctic tundra, and on the International Shorebird Survey for trends in shorebirds such as sandpipers and plovers. Aerial surveys of water bodies, swamps, and marshes filled out the picture for waterfowl. All together, they studied 529 bird species, about three-quarters of all species in North America, accounting for more than 90% of the entire bird population.
"I frankly thought it was going to be kind of a wash," Rosenberg says. He expected rarer species would be disappearing but common species would be on the rise, compensating for the losses, because they tend to be generalists, and more resilient. Indeed, waterfowl and raptors are thriving, thanks to habitat restoration and other conservation efforts. But the declines in many other species, particularly those living along shorelines and in grasslands, far exceeded those gains, Rosenberg and his colleagues report. Grassland birds have declined by 53% since 1970—a loss of 700 million adults in the 31 species studied, including meadowlarks and northern bobwhites. Shorebirds such as sanderlings and plovers are down by about one-third, the team says. Habitat loss may be to blame.
The familiar birds that flock by the thousands in suburbs were not exempt. "There's an erosion of the numbers of common birds," Rosenberg says. His team determined that 19 common species have each lost more than 50 million birds since 1970. Twelve groups, including sparrows, warblers, finches, and blackbirds, were particularly hard hit. Even introduced species that have thrived in North America, such as starlings and house sparrows, are losing ground.
"When you lose a common species, the impact will be much more massive on the ecosystem and ecosystem services," says Gerardo Ceballos, an ecologist and conservation biologist at the National Autonomous University of Mexico in Mexico City. "It's showing the magnitude of the problem."
Some of the causes may be subtle. Last week, toxicologists described how low doses of neonicotinoids—a common pesticide—made migrating sparrows lose weight and delay their migration, which hurts their chances of surviving and reproducing. Climate change, habitat loss, shifts in food webs, and even cats may all be adding to the problem, and not just for birds. "There's general ecosystem collapse that could be happening here," Marra says.
Weather radar data revealed similarly steep declines. Radar detects not just rain, but also insect swarms and flocks of birds, which stand out at night, when birds usually migrate. "We don't see individual birds, it's more like a big blob moving through airspace," explains Cornell migration ecologist Adriaan Dokter. He converted "blobs" from 143 radar stations into biomass. Between 2007 and 2017, that biomass declined 13%, the Science paper reports. The greatest decline was in birds migrating up the eastern United States. "It's an independent data set that confirms the other work," says Nicole Michel, a population biologist with the National Audubon Society's Conservation Science Division in Portland, Oregon.
"We want this to be the real wake-up call," Rosenberg says. The recovery of eagles and other raptors after the U.S. ban on the insecticide DDT in 1972 shows that when the cause of a decline is removed, "the birds come back like gangbusters." This time around, reversing habitat loss—from the conversion of grasslands for biofuel crops or coastal development, for example—could help stabilize populations.
Concurrent with the paper, a coalition of conservation groups has come up with policy recommendations and an action plan for citizens. Simple steps, such as keeping cats indoors or planting native plants, can help, Rosenberg says. "I am not saying we can stop the decline of every bird species, but I am weirdly hopeful."