New research led by the American Museum of Natural History suggests that there are about 18,000 bird species in the world - nearly twice as many as previously thought. The work focuses on "hidden" avian diversity - birds that look similar to one another, or were thought to interbreed, but are actually different species. Recently published in the journal PLOS ONE, the study has serious implications for conservation practices.

Birds are traditionally thought of as a well-studied group, with more than 95 percent 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's known as the "biological species concept," which defines species in terms of what animals can breed together.

The researchers also surveyed existing genetic studies of birds, which revealed that there could be upwards of 20,000 species. But because the birds in this body of work were not selected randomly -- and, in fact, many were likely chosen for study because they were already thought to have interesting genetic variation -- this could be an overestimate. The authors argue that future taxonomy efforts in ornithology should be based on both methods.

Increasing the number of species has implications for preserving biodiversity and other conservation efforts.