Recently, two distinct but closely related wren species were recognized in North America: Pacific Wren (Troglodytes pacificus) of the West; and Winter Wren (Troglodytes hiemalis) from the East. This means that birders are now reporting two different species of wrens in North America when just a few months ago they were all known as a single species: Winter Wren Troglodytes troglodytes.
The STEM map for this species-pair was generated before the split, so it shows both of the current species in a single model (we now refer to this population as Pacific/Winter Wren). In fact, seeing them both on the same map is a great way to test just how well STEM is differentiating distinct regional populations. Different species presumably have different habitat requirements, migration patterns, and ranges, and this is certainly true of Pacific Wren and Winter Wren. Pacific Wrens breed in cool evergreen forests of the western US and Canada and are present year-round in many areas of their range, such as northern California and the Pacific Northwest. Winter Wrens breed in deciduous and evergreen forests in the Appalachians and northern United States from Minnesota eastward, and they winter across the southeast in deciduous swamps and dense tangles. If STEM works as it is should, and truly uses regional models that are then aggregated for a larger map (rather than trying to understand the occurrence of a “species” across its range using the same variables), then it should produce a model that looks quite different in the East and the West. And it does. In fact, if you looked at this map running you might surmise that the two populations were completely different!
On the STEM animations above, it is clear that there is a sharp divide between these two wrens that is centered on the Great Plains. This area, where eastern bird populations give way to western bird populations, is a typical species-level boundary. Think of all the species that have eastern and western counterparts with divisions that occur in this area: Black-headed and Rose-breasted Grosbeaks, Scarlet and Western Tanagers, Eastern and Western Wood-Pewees, Indigo and Lazuli Buntings, Eastern and Spotted Towhees, and many more. This alone may be an indication that two species are involved, since this so closely matched the biogeographical boundaries of other North American species. (Myrtle and Audubon’s forms of Yellow-rumped Warbler follow this boundary too, but have yet to be formally split).
There are a number of other general differences. Notice how strongly migratory Winter Wren is when compared with Pacific Wren. The eastern species almost entirely withdraws from its northern breeding areas and winters in a broad area of the Southeast, without any real structure to the wintering area. The Pacific Wren, by contrast, shows obvious structure to both the winter and summer range (which are very similar). This structure corresponds to broad habitats where forests occur in the West; the desert areas show up as black or very pale orange (indicating low occurrence), while the “hot” areas invariably correspond to areas of forests, usually mountains.