According to a new study, there are at least 50 billion total wild birds—or six birds for every human on the planet.
The new paper, published this month in the journal Proceedings of the National Academy of Sciences, examined data collected by users of the citizen science database eBird and compared it with data collected by Partners in Flight and BirdLife International to check the accuracy. The researchers then combined the three pools of data to create an algorithm that estimated the population sizes for other species that were not the subject of the professional studies and lacked significant data, reports Adam Vaughan for New Scientist.
"The really big breakthrough in this paper was we could take the scientific data and the citizen science data and then fill the gap for birds which are not studied by professional scientists," says ecologist and co-author Will Cornwell of the UNSW to Lisa Cox for the Guardian.
Out of all 9,700 species analyzed, four birds reached what the researchers call the "billion club," or species with an estimated global population of over a billion. These included the house sparrow at 1.6 billion, the European starling at 1.3 billion, the ring-billed gull at 1.2 billion, and the barn swallow at 1.1 billion.
In contrast, 1,180 species, or 12 percent of the population numbers, have fewer than 5,000 individuals, reports New Scientist. Some of these species included the great spotted kiwi at 377 individuals, the Javan hawk-eagle at 630 individuals, and the Seychelles kestrel with under 100 individuals remaining, per Douglas Main for National Geographic.
"It's really ambitious—it's a big undertaking to try and figure out how many birds there are in the world. They thought really deeply about it and took as many steps as possible to make it as precise as possible," says Lucas DeGroote, a researcher at the Powdermill Avian Research Center at the Carnegie Museum of Natural History, to National Geographic.
When the last review of bird populations was taken 24 years ago, estimates reached between 200 billion and 400 billion individuals, New Scientist reports. The vast discrepancy between the old and new data reflects different methods used for analysis and data collection; it does not reflect an overall decline in birds, reports New Scientist.
Overall, the study helps provide a baseline for researchers interested in future bird populations' calculations, improve conservation efforts of rare species, and help researchers find what makes a rare species scarce, study author Corey Callaghan, a biologists at the University of New South Wales, explains to New Scientist.
"Quantifying the abundance of a species is a crucial first step in conservation. By properly counting what's out there, we learn what species might be vulnerable and can track how these patterns change over time - in other words, we can better understand our baselines," says Callaghan in a statement.