The Disappearing Birds

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 reported in the September 2019 Issue of Science Magazine…  Five  years ago, PM Rosenberg a conservation biologist decided to take a broader look at what is happening in North America’s skies.

“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. 

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….

Excerpts from Elizabeth Pennisi, Billions of North American Birds Have Vanished,  Science, Sept. 20, 2019

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