The American Ornithologists Union just released a new Supplement to the Check-list of North American Birds, an event that has birders scurrying to update their lists as species, genera, and sometimes higher taxa are split, combined, and reorganized based on the latest scientific data. But what kind of data convinces the authorities at the AOU that the checklist needs changing?

One of the most interesting changes in the 51st Supplement is the splitting of the species formerly known as Winter Wren (Troglodytes troglodytes). The case for the split was made in a recent in the journal Molecular Ecology by David Toews and Darren Irwin of the University of British Columbia. The Winter Wren is a widespread species, found in temperate forests throughout North America and Eurasia. At last count, avian taxonomists had classified 44 geographic subspecies of the Winter Wren, each a subtle variant in plumage or morphology.

But some variation is more striking: for example, there are major differences in song between the Pacific (T. t. pacificus) and eastern (T. t. hyemalis) subspecies. Toews and Irwin wanted to know if this song variation could cause reproductive isolation between these subspecies – if their songs were sufficiently different, the two subspecies might not interbreed if they came into contact. Species are often defined according to the Biological Species Concept, which follows Ernst Mayr’s definition of species: “groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups.” So if Toews and Irwin were right and the eastern and Pacific Winter Wrens really didn’t interbreed where their ranges overlapped, then they wouldn’t just be subspecies – they would be two different species.

The first step was to find a site where both subspecies occurred; no such locations were known before their study, but Toews and Irwin eventually found such a place in northeastern British Columbia. Then, they captured adult wrens, took blood samples for DNA analysis, and recorded males singing.

Even though males singing hyemalis-type song and pacificus-type song were often found in adjacent territories, no male sang both singing types. Moreover, the two singing types could be easily and reliably distinguished using acoustic analysis. At the sympatric site – i.e. the site where both species occurred – the two singing types were just as distinctive as were songs recorded in geographically distant populations of the eastern and Pacific-type wrens.

Next, Toews and Irwin analyzed the mitochondrial DNA of the wrens they captured, and found that the DNA sequences were quite different between the two “subspecies,” but nearly invariant within them. After calibrating the observed genetic divergence with a molecular clock – a rate of DNA sequence evolution calculated from known divergence times between species – they suggest that the two “Winter Wrens” may have diverged from one another up to 4.3 million years ago, before the Pleistocene glaciation events that are thought to have driven the diversification of many North American bird groups.

What does all of this mean? For the wrens, not much! They’ll go on doing what they’ve been doing for millions of years, regardless of what we call them. But the AOU has considered the evidence that Toews and Irwin collected and taken action: the Winter Wren (Troglodytes troglodytes) is now split into the Winter Wren (T. hyemalis) in eastern North America and the Pacific Wren (T. pacificus) in western North America. Since I’ve seen The Bird Formerly Known as Winter Wren on both sides of the continent, I suddenly have another species on my life list!

Based on other evidence, the AOU also split both North American species from the newly-named Eurasian Wren, which (because it was the first form described by scientists) keeps the original Latin name Troglodytes troglodytes. The fact that we are only just recognizing species boundaries in such a well-studied bird means that there are surely other “cryptic species” out there, just waiting to be discovered!