WC is, of course, a birder. And keeps a list of the birds he has seen, which makes WC a “lister,” WC supposes. Not hard core. When a rare Asian bird would turn up in Gambell, Alaska, out in the Bering Sea, WC didn’t drop everything and rush out to see it, to add it to his list. But WC and Mrs. WC do plan their vacations around birding. And as a result, WC has seen about 20% of the world’s 10,000 or so birds.1 Decent, but hardly noteworthy. Hardcore listers have 8,000 species or more. But WC had a seen respectable number of species.
Or so WC thought.
Now those pesky ornithologists are coming around to thinking there may be as many as 22,000 species of birds in the world.
Who moved WC’s cheese?
George F. Barrowclough, Joel Cracraft, John Klicka and Robert M. Zink, that’s who. In a paper titled, “How Many Kinds of Birds Are There and Why Does It Matter?” published in Plos One back in 2016, the researchers took two approaches to re-counting the number of bird species. In the first, they took a sample of 200 species of birds and studied their morphology, the details of their shape, colors and forms, and used the differences they found to re-count the number of species. And arrived at 397 species. That’s an average of 1.97 species where birders saw just one. In 59% of the 200 birds species, the researchers agreed there was just one species. But in the other 41% of those 200 species, the researchers found multiple species. That was the morphology analysis.
The second approach was molecular genetics. Simplifying a bit, it looks at the mitochondrial DNA (mtDNA) of the birds and studies the variations. Using that approach, there were even more species, perhaps as many as 24,000. The mtDNA is admittedly a little more suspect; the birds studied weren’t selected at random. But still.
So what’s going on? Why is the official number of 9,000 – 10,000 so different from the 20,000 – 25,000 the researchers found?
Ornithology generally uses an older definition of “species” than taxonomists do generally. For ornithologists, the test is whether the birds can breed and produce viable, breeding offspring. Even that’s a little soft: gulls, for example, interbreed. Western Gulls, in particular, breed with other species,especially Glaucous-winged Gulls, and their hybrid offspring torment birders who try to identify them. Ornithologists classify morphological differences as subspecies, not species. The regional variations in species like Fox Sparrows, or Yellow-rumped Warblers, or dozens of other instances, are treated by ornithologists as subspecies. The researchers use a different approach.
So the difference between the traditional count of bird species and the study’s conclusions is a matter of approach. Barrowclough et al. make the case for phylogenetic analysis, and, effectively, more than doubling the number of species. And they have the tide of biology running with them. After all, it is genetics that determine it all.
The answer, then, is that biology is moving WC’s cheese.
- There’s no agreement on the number of bird species on the planet. The International Ornithological Congress recognizes 10,711 species. The highly regarded Clements Checklist has 10,404 species, of which 101 are listed as extinct. The American Ornithologists’ Union (AOU) says 9,159. So, yeah, the weasel words “or so.” ↩