Until recently, scientists believed there were about 9,000 species of birds on earth. Now,as Quartz.com reports,it looks like the number is closer to double that—around 18,000.
So, did researchers just discover a slew of birds hiding in the wild? No, they actually did a recount, using a more nuanced definition for a species.
Take the spotted owl, for instance. These North American birds used to be considered one single species. Now, as ornithologist Joel Cracraft explains, “there are actually three different kinds of spotted owl.” There’s a western Canadian species, one in the western US, and one that dwells in Mexico. While these three groups are physically capable of having chicks together, it’s not likely to happen often. Spotted owls don’t migrate, and they mate for life. So, even though the birds hold deep similarities, the fact that they don’t interact means bird scientists consider them separate species.