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Tūī make up 2.3 million of global bird population of 50 billion, research project finds

The kiwi has been found to be the rarest order of birds in an Australian research project that's concluded there are roughly 50 billion birds in the world, thanks the input of over 600,000 citizen scientists.

Tui bird. Source: Supplied

The study by the University of NSW suggests there are about six birds for every human on the planet.

The findings are based on the observations of dedicated bird watchers and detailed algorithms which estimate how many birds belong to 9700 different species, including flightless birds like the kiwi.

North Island brown kiwi chick. (File photo) Source: istock.com

Researchers say the North Island brown kiwi, the southern brown kiwi and the great spotted kiwi — with estimated populations of 1833, 729, and 377 respectively — were the rarest order of birds.

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The southern brown kiwi is under threat from stoats on the West Coast. Source: 1 NEWS

Conversely, the study found there were an estimated 2.3 million tūī.

Little spotted kiwi chick. Source: Andrew Digby / DOC

Many iconic Australian birds were also found to number in the millions, including the rainbow lorikeet (19 million), sulphur-crested cockatoo (10m) and kookaburra (3.4m).

"Humans have spent a great deal of effort counting the members of our own species - all 7.8 billion of us," Associate Professor Will Cornwell, a co-senior author of the study said.

"This is the first comprehensive effort to count a suite of other species."

The research team pooled almost a billion bird sightings logged on eBird, an online database of bird observations from citizen scientists.

They then developed an algorithm to estimate the actual global population of each bird species.

The calculation took into account each species' "detectability" - how likely it is that a person will have spotted this bird and submitted the sighting to eBird. Detectability can include factors such as bird size, colour, whether it flies in flocks and if it lives close to cities.

Lead author Corey Callaghan said the large-scale data integration approach of the study could act as a blueprint for calculating species-specific abundances for other animals.

"Quantifying the abundance of a species is a crucial first step in conservation," he said.

"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."

The data includes records for 92 per cent of bird species with the remaining eight per cent excluded for being so rare.

Four bird species were found to have an estimated global population of over a billion. The house sparrow has the most with 1.6 billion, while the European starling (1.3 billion), ring-billed gull (1.2 billion) and barn swallow have populations 1.3 billion, 1.2 billion, and 1.1 billion respectively.

While some bird populations are thriving, around 12 per cent of bird species included in the study have an estimated global population of less than 5000.

These include species such as the Chinese crested tern, noisy scrub-bird, and invisible rail.

"We'll be able to tell how these species are faring by repeating the study in five or 10 years," Cornwell said.

"If their population numbers are going down, it could be a real alarm bell for the health of our ecosystem."

The findings are published in The Proceedings of the National Academy of Sciences.

AAP contributed to this report.