There are about 50 billion birds in the world, according to a new study that uses citizen science observations to try to estimate the population of nearly 10,000 species.
The study, led by scientists at the University of New South Wales in Australia, suggests there are about six times as many birds as humans on the planet - but many species are very rare.
Four species belong to what the researchers call the "billion club," with a total bird population estimated at more than one billion, the report said. They are sparrows, European starlings, ring-billed gulls and house swallows found in many parts of the world.
Researchers estimated 9,700 species, including penguins, emus and kookaburra, using hundreds of millions of bird observations recorded by birders on eBird, one of the world's largest citizen science projects on biodiversity.
They combined their records with professional scientific observations to form an algorithm that can estimate the population size of almost any species, the report said. The team of scientists found relatively few common birds, but a large number of rare species.
"They could be rare for natural reasons - like they only actually live on an island or on a mountaintop - or they could be rare for human reasons," said Will Conwell, an ecologist at the University of New South Wales and one of the paper's lead authors.
Over time, he says, he hopes the models will show which species are in decline and where conservation needs to be implemented.
Sean Dooley, national public affairs manager for Bird Life Foundation Australia, is a long-time birder who makes his contribution to citizen science programs. He says the paper demonstrates the value of citizen observation's contribution to scientific research.
"This is a great first step in trying to figure out what we have," he said. "If we can keep doing this -- it's going to be a really important thing, because we know we're seeing a massive loss of wildlife. Being able to get close to quantifying what's going on is critical."
By combining bird-watching with professional records of bird monitoring, the scientists were able to build a model, Conwell said. The model will then be applied to birds that are not the subject of specialist research.
The 50 billion figure is the model's median estimate for all the world's birds, the report said. There is some uncertainty in the numbers, Conwell said, and the researchers plan to improve the model as more specialized studies are conducted on more species.