Bird-watchers of the world: your list may have just doubled. A recent study published in the journal PLOS One suggests that the way bird species are defined has traditionally been too narrow, and there are actually more than 18,000 bird species in the world—double previous estimates.
Aside from making it trickier for bird fans to tick off all the species on their list, how we classify bird species has serious implications for conservation. If a bird is thought to be a subspecies of a healthy population, but is really a tiny population of its own species, it could be at risk of extinction without anyone realizing. That’s what happened recently with the red-bellied pitta, which was believed to be a single species but turned out to be 13 different species of bird, some of which were threatened, while others were of least concern.
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“Under the International Union for Conservation of Nature criteria, any species that has a population under 1,000—whether that population is increasing or whether there is an immediate threat or not—it’s regarded as intrinsically vulnerable to extinction,” said Nigel Collar, a Leventis Fellow in conservation biology with BirdLife International.
The PLOS One study took a different approach to determining if a species is unique, or is a subspecies of another bird. Rather than relying on whether or not two birds can breed as a determination of how similar or distinct they are—one of the classic ways of distinguishing species—researchers instead focused on genetic differences and morphology (the physical characteristics of the birds, like feather color and beak shape).
For the morphology approach, the researchers investigated a sample of 200 species and found close to two separate species within each of the currently recognized species. They found similar results when looking at the genetic differences between species.
“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, in a press release. “This new number says that we haven’t been counting and conserving species in the ways we want.”
Within the world of conservation biology there is some debate over the best way to determine what makes a species a species, so this new estimate won’t necessarily be accepted by everyone. But it does raise some compelling questions over whether our current standards are accurate enough, and when it comes to birds on the brink, it’s a question that could mean the difference between extinction or survival.
