Scientists knew that the numbers of some species of birds were declining. But they also knew some were increasing in number, and thus they weren't sure there had been a net loss. So they collected numbers from surveys of 529 bird species over the years, along with radar information, to arrive at a total count, NPR reports. Now they know: The data showed that since 1970, North America has lost 3 billion birds—29% of the total a half-century ago. The reduction is widespread. "We saw this tremendous net loss across the entire bird community," said an applied conservation scientist. Most of the decline involves sparrows, warblers, blackbirds and finches, and other common families. More than one-third of shorebirds are gone, and grassland birds have declined by more than half since 1970.
A biologist not involved in the research called the findings, published Thursday in Science, "depressing but not surprising." The grassland birds' number wasn't shocking because of pesticides and changes to their habitats caused by humans, per the Washington Post. But data about other, more familiar birds was "staggering," one scientist said. “The generalist, adaptable, so-called common species were not compensating for the losses, and in fact they were experiencing losses themselves," he said. We know more about the situation with birds because they're easier to count than, say, insects. So this decline could be a sign of a wider problem, scientists say. "These birds are an indicator of ecosystem health," said a London professor, adding, "Studies like this do suggest the potential of a systems collapse." (A recent study found plant species disappearing.)