This was a perfect opportunity to finally compare these two woodpeckers, side by side. I grabbed my trusty Sibley Field Guide to finally settle this pressing matter. See, they look almost identical, save for a few crucial details.
Downy Woodpecker by Peggy Collins, via flickr
Hairy Woodpecker by Geoff Patterson via flickr
The big difference is that the Hairy is larger - not evident in the photos, unfortunately - with a more robust beak and plain white feathers under the tail. The Downy has a thicker tuft of white feathers on top of the beak, and the feathers under the tail have black spots. The above birds are both females; males of both species have a red band on the back of the head. Not being an expert birder by any means, I used to just call any black and white speckled woodpecker I saw flying around a Downy and be done with it. Seeing both in close proximity was a huge help. It made my morning!
So let's pretend that I live near a volcano, and today it erupts, burying these woodpeckers, me, and everything else in my yard. Fast forward ten million years or so. Incredibly, there are still paleontologists around, and they dig up these woodpeckers. Considering how similar the birds are, would the future paleontologists ascribe them to different species? Would they instead be thought to represent natural size variation or different ontological* stages within the same species?
With fully preserved skeletons, a skilled paleontologist could probably deduce that they are different species, at the very least based on the beak. I can only assume that the post-cranial skeleton is similar enough that if the head was missing, it would create quite a challenge. I could certainly see it being debated.
The funny thing is that based on recent genetic sequencing, it appears that the Downy and Hairy woodpeckers, both now placed in the Picoides genus, may need to be placed in separate genera. I had planned on summing up their relationship by noting that they share a recent common ancestor, but based on the distinct selective pressures placed on to separate populations, the lineages leading to Downy and Hairy woodpeckers split. When I did a bit of research, it turned out not to be quite so simple. Their ancestor is more distant than I'd assumed.
It's one of the consequences of technology. We now have to tools to study organisms at a deeper level than outward features, and science cannot ignore new information. It's less of an issue with living species: they have common names unrelated to the latin binomial. When we see chimpanzees we call them "chimpanzees," not Pan troglodytes. Unless we're unsufferable nerds, of course. But when the same thing happens to genera of dinosaurs and other extinct creatures, it's a bigger problem. Thus, Brontosaurus is learned to be a false genus based on a skull being placed on the wrong body. The world still isn't used to Apatosaurus.
*Ontogeny = the way an organism changes from birth to adulthood.