The number of known bird species in the world is only half of the actual number according to the American Museum of Natural History. A new study featured in the journal PLOS ONE suggests that the actual number of bird species actually total up to about 18,000 when "hidden" avian diversity is factored in.

What had previously been considered species that interbred or simply looked similar to each other are now being marked as different species and this new way of surveying birds may have serious consequences for avian conservation.

"We are proposing a major change to how we count diversity," said Joel Cracraft, an author of the study and a curator in the American Museum of Natural History's Department of Ornithology. "This new number says that we haven't been counting and conserving species in the ways we want."

Over 95 percent of bird species are identified globally, with the number of recorded species reaching up to 9,000 to 10,000 as reported by bird watchers and scientists. That estimate, however, is based on "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"It's really an outdated point of view, and it's a concept that is hardly used in taxonomy outside of birds," said lead author George Barrowclough, an associate curator in the Museum's Department of Ornithology.

As part of their new research, Cracraft, Barrowclough, and their colleagues at the University of Nebraska, Lincoln, and the University of Washington studied random samples of 200 bird species using morphology, or by examining the physical characteristics like plumage pattern and color, which can be used to highlight birds with separate evolutionary histories. The result? On average, there were nearly two different species for each of the 200 birds studied. Given these findings, it could be concluded that bird biodiversity is severely underestimated and that there were likely 18,000 bird species worldwide.

"We have decided societally that the target for conservation is the species," said Robert Zink, a co-author of the study and a biologist at the University of Nebraska, Lincoln. "So it follows then that we really need to be clear about what a species is, how many there are, and where they're found."