Around 50 billion birds live on Earth, meaning that there are around six times as many birds as humans on the planet, according to a study by Australian scientists.

The research, published in the journal Proceedings of the US National Academy of Sciences, worked with data on 9,700 known bird species from around the world, which they said represented 92% of all extant species.

"Quantifying abundance, however, is difficult and time consuming," the study said. The researchers combined scientific surveys of individual species across specific distribution areas, with almost a billion entries posted in the online ornithological database eBird.

"We conclude that there are many rare species, highlighting the need to continue to refine global population estimates for all taxa and the role that global citizen science data can play in this effort," the team, led by William Cornwell from the University of New South Wales in Sydney, said.

"There are many rare species and comparatively few common species," they said. "By aggregating the species-level estimates, we find that there are ∼50 billion individual birds in the world at present."

Which birds are most common?

Among the top most abundant birds in the world are the house sparrow (1.6 billion), the European starling (1.3 billion), the ring-billed gull (1.2 billion) and the barn swallow (1.1 billion). Meanwhile, the most abundant orders of birds were perching birds (28 billion), shorebirds (9.7 billion), and birds commonly found along shorelines and mudflats, or waterfowl (2.3 billion).

The study found that the least-abundant types of birds in the world were flightless kiwis from New Zealand (3,000) and mesites (154,000), scrubland birds typically found in Madagascar.

Researchers found that the majority of the birds are from the palearctic and nearctic biogeographic realms, which together include most of North America, Asia and Europe.

"Quantifying the abundance of species is essential to ecology, evolution, and conservation," the researchers said. "Yet the empirical pattern at the global scale remains unresolved, with a few species' abundance well known but most poorly characterized."