There are 50 billion wild birds on Earth – but four species dominate – says a New Scientist article on 17 May 2021.

Earth has around 50 billion wild bird species according to a new global estimate, but most species are very rare and only a handful number in the billions.

Just four wild species have over a billion individuals, and they are the most common wild bird species in the world. This is in contrast to 1,180 species that have less than 5,000 individual birds each.

House Sparrows (Passer domesticus) are the most abundant, followed by European Starlings (Sturnus vulgaris), Ring-Billed Gulls (Larus delawarensis), and Barn Swallows (Hirundo rustica). 

“One of the takeaways is mother nature just loves rare species. It’s what some refer to as hyper dominance, which has been found in Amazonia tree flora and other plant groups. It’s not terribly surprising, but it’s good to have the data,” says Corey Callaghan at the University of New South Wales in Australia, who led the research.

The estimate of around six wild birds for every human on the planet is the first since researchers arrived at a global figure of 20-400 billion wild birds in 1997, 24 years ago. The big gap between the studies isn’t due to a dramatic decline in bird numbers, but is explained by a more sophisticated method that used data to count more species.

Callaghan and his research colleagues took citizen science data on bird sightings from the online eBird database to build a model that estimated global numbers for each species. To ensure it was working well, they cross-checked the results for 724 species with other rigorous data sources on well-studied birds. The model was then extrapolated out to 9,700 species, arriving at a median of 50 billion wild birds globally.

The citizen science sightings are both a strength and a weakness of the research, says Richard Gregory at the UK charity the Royal Society for the Protection of Birds. Richard Gregory says that the eBird database has far less data on birds in the tropics than in temperate regions. For instance, the Red-Billed Quelea is sometimes regarded as the most numerous wild bird species on the planet, but in the new analysis its population is estimated at just 95 million.

Another example is that the model initially predicted a best estimate of about 500 Ivory-Billed Woodpeckers, although it is thought to be extinct. That quirk was caused by two erroneous sightings of the species that were entered in the database when it was downloaded, which were later removed by reviewers. Nonetheless, Callaghan says such granular differences for individual species don’t change the overall estimate.

Callaghan says we could improve bird conservation with further research on why some species are rare, whether it is just because they have evolved to occupy a single island or because of human activities such as deforestation.

Journal reference:PNAS, DOI: 10.1073/pnas.2023170118

Photographer: Martina Nicolls

Martina Nicolls:SIMILAR BUT DIFFERENT IN THE ANIMAL KINGDOM