There are at least 50 billion individual wild birds in the world, according to a new estimate.
House sparrows alone make up about 1.6 billion of these.
And three other species - European starlings, barn swallows and ring-billed gulls - also have populations exceeding one billion birds.
However, most bird species are rare, with about one-in-ten species down to fewer than 5,000 individuals, say the authors.
This "snapshot" of the global bird population will help in conservation efforts to save birds from extinction, says a team at the University of New South Wales in Sydney, Australia.
"We spend a lot of time and money counting our own species, but we really need to think about how we count the biodiversity that we share the planet with," study researcher Dr Corey Callaghan told BBC News.
Counting the number of birds in the world is a complex task, with no definitive answers.
Past rough estimates have come up with 200 to 400 billion individual birds drawn from 10,000 to 13,000 bird species.
The Australian researchers analysed 9,700 species of living birds (excluding all domestic birds) using data recorded by birdwatchers on the online database, ebird, over the past decade.They refined the data using modelling and information from experts on the ground to come up with what they say is a more accurate estimate.
The study, published in the Proceedings of the National Academy of Sciences, suggests most birds are found in the northern hemisphere: in Europe, northern Asia, northern Africa, parts of the Arabian Peninsula, and North America.
In contrast, very few birds are found in Madagascar and the Antarctic.
"The citizen science data will play a fundamental role in the future in biodiversity monitoring," said Dr Callaghan.
