New York, Dec 13 (IANS): There are about 18,000 bird species in the world - nearly twice as many as previously thought, new research suggests.

"This new number says that we haven't been counting and conserving species in the ways we want," said one of the study authors, Joel Cracraft, from American Museum of Natural History.

Birds are traditionally thought of as a well-studied group, with more than 95 per cent of their global species diversity estimated to have been described.

Most checklists used by bird watchers as well as by scientists say that there are roughly between 9,000 and 10,000 species of birds.

But those numbers are based on what is known as the "biological species concept," which defines species in terms of what animals can breed together.

"It's really an outdated point of view, and it's a concept that is hardly used in taxonomy outside of birds," lead author George Barrowclough, an associate curator in the Museum's Department of Ornithology.

The work focuses on "hidden" avian diversity - birds that look similar to one another, or were thought to interbreed, but are actually different species.

"We are proposing a major change to how we count diversity," Cracraft, said

For the new work, the researchers examined a random sample of 200 bird species through the lens of morphology - the study of the physical characteristics like plumage pattern and colour, which can be used to highlight birds with separate evolutionary histories.

This method turned up, on average, nearly two different species for each of the 200 birds studied.

This suggests that bird biodiversity is severely underestimated, and is likely closer to 18,000 species worldwide, said the study published in the journal PLOS ONE.