Cryptic Species

One of the things that makes birding interesting as a hobby in a way you might not expect is that the collective ornithological knowledge upon which all birding is based - field guides, checklists, etc. - is always changing. That's because there is still a whole lot of mystery in the natural world, and every once in awhile, someone unlocks a new door and discovers something exciting. For example, there are still new species being discovered right here in North America.

How can that be? Haven't we exhaustively explored every inch of our continent? Well, yes, but sometimes two different species can look identical. All North American birds have been named via a dead bird - a "type specimen" in a dusty specimen cabinet in a university or a library shot by famous old ornithologists birds are named after - like Nuttall, Audubon, or Townsend. So they were named by appearance - the physical make-up (or morphology) of the bird.

But, if they are genetically distinct despite their identical appearance, and choose not to breed with each other (more often than not, given the choice), than isn't that a different species? The most famous examples of this is found in the Tyrannid (New World Flycatchers) family - these birds do not learn their songs, like most singing birds, but rather are genetically programmed to sing the songs they sing. So, even though some are basically identical (Alder and Willow Flycatchers, Cordilleran and Pacific Slope Flycatchers, Eastern and Western Wood-peewees), if they have different calls, that reflects a genetic difference. And, if the birds choose to breed with their own species instead of their sister species when given the choice (this happens where the species are found in the same place - so called "contact zones"), then they're separate species. Since female birds are apt to choose their mates based on song, this is what does happen, and so we've been adding species since people started studying this sort of thing in the last 30 or 40 years.

A recent book I read (The Singing Life of Birds by ornithologist Donald Kroodsma) suggests that Winter and Marsh Wrens are distinct species (both with an eastern and western version) based on the fact that they have distinct songs and calls. He did laboratory studies where he raised eastern and western wrens in the same room, playing eastern and western songs for them. He found that the birds, though song-learners, didn't learn each others' songs as well as they learned their own version of Marsh Wren song. The author then studied the contact zone for Marsh Wrens in the Midwest, and found that the birds maintained their perfectly distinct songs even when found in the same marsh. His conclusion is that, given sexual selection for birds that sing familiar songs and the genetic predisposition to learn their own song, the birds are distinct species. This probably needs to be followed up with a genetic study to confirm the females' preference before it will be accepted, but it seems very likely.

He had not found or studied a contact zone for Winter Wrens, however. Winter Wrens are, to the English, THE Wren. It's the only kind of Wren found in the "old world", and it's found from the Bering Straight to the British Isles - in both hemispheres. The Pacific-slope population, though, has long been thought to be the most accomplished singer. It sings longer and more complex songs than any other group. Finally, someone's done the study, and found the place where this complex singing group meets their more widespread (and less virtuoso) eastern relative. And, lo and behold, the birds in the contact zone maintain their distinct song, defending neighboring territories in the coniferous forests of western Canada. And, the molecular DNA study shows they don't interbreed. So, the females are choosing the birds that sing the song their ancestors sung, and the birds are therefore a distinct species. But you can't tell them apart just by looking!

Winter Wren from Germany (Wikipedia)
This bird is showing the distinctive Winter Wren formula - a little mousey brown bird with a short barred tail and an eye-brow. Looks like a Bewick's Wren whose tail's cut off. The Pacific Winter Wren looks pretty much the same.

So the Winter Wren Libby and I've seen (in Northern California) turns out not to be Troglodytes troglodytes. It's Troglodytes pacificus - not officially, there's a group (the AOU) that decides that, but almost certainly. Now, I would be interested to see if the birds of Europe are the same as the birds in eastern North America. There aren't any contact zones, so people will have to come at it from a different angle. Hopefully some ornithologist will pursue it, now.

For birdwatchers, there'll be a new bird to identify, tick, add to our lists. A new vagrant to look for in the Fall (an eastern Winter Wren was in Huntington Beach this Fall, for example, identified by call notes). A new bird to seek out in the Northeast. Since the whole pleasure of birdwatching hinges on identifying birds, this carries significance. The field guides will be have to be amended, with new color-coded range maps and illustrations. Libby and I's current field guides are already lacking the Cackling Goose and the Dusky vs. Sooty Blue Grouse split, so this is just one more thing to learn. I love that things are so dynamic. What larks!