North America has lost almost 3 billion birds since 1970, according to a new bird survey analysis and radar data from across the continent.
The sharp decline, described in a study published Thursday in the journal Science, it’s not just bad for birds. It also bodes ill for the ecosystems these birds inhabit, and points to a need for action to halt and perhaps reverse the fall, the scientists said.
Many animals are threatened with extinction because of human activity, but this is not the only way to harm them. Loss of abundance also creates problems as it can have profound consequences for the ecosystems in which they live.
Conservationists studying birds in North America knew that at least some species were declining, but they did not know what the net loss or gain of all birds would be.
“Previously, we didn’t have good estimates of population size,” Rosenberg said. “We knew the trends, but we didn’t know how many birds of each type were there.”
To find out, he and his colleagues analyzed more than a dozen bird research datasets covering 529 bird species across a range of ecosystems in the US and Canada. These data sets, such as the Breeding Bird Research in North America, they rely on citizen scientists who spread along US roads every year to do their counting. The records date back decades.
The researchers were also able to track feathered flyers with a network of 143 weather radar sites, which often catch migratory birds on their nightly routes.
The data came to a grim conclusion: For nearly half a century, bird populations in North America have declined sharply. Today there are 2.9 billion fewer birds than in 1970 – a 29% reduction.
Steven Weissinger, a UC Berkeley biologist, called the results and their implications “stunning.”
“I was quite surprised,” said Beissinger, who was not involved in the study. “We don’t usually think of billions of birds.”
Of these lost birds, 90% came from only 12 bird families that include frequent and common species such as sparrows, swallows, warblers, and finches.
The decline in abundance of common species may not seem as dramatic as the danger of rare species, but it is a severe form of ecosystem erosion, the scientists said.
This is because abundant species often play essential roles in their biomes, whether they control pests, pollinate flowers, disperse seeds, provide food for other animals, and even contribute to the natural beauty of an area that attracts tourists who support local economies.
“When you are losing abundance, you are losing the tissue of food chains, the tissue of ecosystems – perhaps more than losing a rare species,” Rosenberg said.
It’s hard to tell which ecosystem services have been lost or degraded due to bird loss in the last half-century, Beissinger said. For example, if there were more birds around to eat insects, farmers could be using fewer pesticides.
Many bird species have experienced dramatic declines in North America since 1970, according to a new study. Clockwise from the top left: a dark-eyed reed, a wood thrush, a chicken beak, and a cactus wren.
(Jay McGowan and Brian Sullivan / Macaulay Library at Cornell Lab of Ornithology; Michael Parr / American Bird Conservancy)
Other previously common species have fallen from mere loss of abundance to elimination.
Rosenberg pointed to the example of the passenger pigeon. It was probably the most abundant bird on the planet, but it was extinct in 1914. He added that the trend line of passenger pigeon losses is similar to the trend observed in the new study, according to the work of one of its co-authors, Jessica Stanton, from the US Geological Survey.
“No one ever thought the passenger pigeon was extinct – and it happened in a relatively short period,” said Rosenberg. “We’re not saying these other birds are on their way to extinction, but it should surely give us a break.”
In all ecosystems, grazing birds – a group that includes sparrows and mallow – were the hardest hit, the researchers said. Since 1970, their numbers have fallen by more than 720 million, representing 53% of the initial population.
Together, more than 1 billion birds have been lost in all forest biomes. Coastal birds, long threatened by coastal wetland drainage and urbanization, have declined by over 37%. Even the ten non-native species counted in the article had a population loss of 63%.
The researchers did not evaluate specific causes for these declines. But Rosenberg said other work pointed to habitat loss due to urbanization, pollution, pesticides, and the intensification and expansion of agriculture as possible culprits.
There were some success stories in the data that could offer a road map to help other bird populations, Rosenberg said. Wetland birds such as ducks and geese have increased, mainly due to conservation efforts that have protected wetland habitats in recent decades. (Much of this conservation was driven by hunters who wanted to keep populations healthy, he added.)
And birds of prey such as eagles, osprey, and peregrine falcons had also improved since the 1970s when the use of Pesticide DDT decimated their numbers. It was banned for agricultural use in 1972, and laws protecting birds of prey allowed them to recover.
“It’s another example of resilience in the bird population itself if we can remove the threats and give them a chance,” Rosenberg said.
A snowy owl.
(Doug Hitchcox / Macaulay Library at Cornell Lab of Ornithology)
Such policies and protections need to be implemented for other birds in other habitats, he said. Hillary Young, a conservation biologist at UC Santa Barbara.
“It’s kind of hopeful because it shows that when you set policies to protect these species, they show long-term recovery trends,” said Young.
But it’s not just some types of birds that are suffering, and she added, “We see widespread suffering.” The breadth of this suffering shows that humans will need to do more to reduce their environmental footprint.
“We’ll have to protect everything better if we want to recover,” she said.
There are things people can do to stop the decline, Rosenberg said. For example, keeping cats indoors, choosing products (such as shade-grown coffee) that do not profit from destroying avian habitat, and planting more bird-friendly native shrubs and trees are small steps that can make a difference.
The next step, Beissinger said, will be to find out which factors are having the most significant impact on bird numbers. “It will be important to try to understand what is causing this,” he said.
Rosenberg, who has been an avid bird watcher since he was 3, said he is choosing to see the new numbers as an opportunity for action.
“It’s time to raise our voice, act and change the way we behave, and finally try to bring about changes in society and politics,” he said. “That’s how I’m thinking.”