North America has lost nearly 3 billion birds since 1970, according to a new analysis of bird survey and radar data from across the continent.

That’s according to a new estimate published in the journal Science by researchers who brought together a variety of information that has been collected on 529 bird species since 1970.

“We saw this tremendous net loss across the entire bird community,” says Ken Rosenberg, an applied conservation scientist at the Cornell Lab of Ornithology in Ithaca, N.Y. “By our estimates, it’s a 30% loss in the total number of breeding birds.”

What makes this study particularly compelling is the trustworthiness of the data. Birds are the best-studied group of wildlife; their populations have been carefully monitored over decades by scientists and citizen scientists alike. And in recent years, scientists have been able to track the volume of nighttime bird migrations through a network of 143 high-resolution weather radars.

This study pulls all of that data together, and the results signal an unfolding crisis. More than half our grassland birds have disappeared, 717 million in all. Forests have lost more than one billion birds.

Their results show that more than 90% of the loss can be attributed to just a dozen bird families, including sparrows, warblers, blackbirds and finches.

Steven Beissinger, a conservation biologist at UC Berkeley, called the results and their implications “dizzying.”

“I was pretty surprised,” said Beissinger, who was not involved in the study. “We don’t usually think in billions of birds.”

Birds are indicator species, serving as acutely sensitive barometers of environmental health, and their mass declines signal that the earth’s biological systems are in trouble. Unfortunately, this study is just the latest in a long line of such mounting evidence.

A study in Germany, for instance, reported a midsummer decline of 82 percent in the biomass of flying insects over the past quarter century. Forty percent of the world’s amphibians are in danger of extinction, according to the International Union for Conservation of Nature. Stocks of bluefin tuna are down to the last 3 percent of their historic population, and the United States’ Atlantic cod fishery recently hit a low.

Elise Zipkin, a quantitative ecologist at Michigan State University, says the loss of individuals can be a big problem.

“Just because a species hasn’t gone extinct or isn’t even necessarily close to extinction, it might still be in trouble,” she says. “We need to be thinking about conservation efforts for that.”

The researchers cite a variety of potential causes for the loss of birds, including habitat degradation, urbanization and the use of toxic pesticides, notes Zipkin.

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